Download 01 - Use of a dDVH decomposition technique to evaluate

01 – USE OF A DDVH DECOMPOSITION TECHNIQUE TO EVALUATE
NORMAL TISSUE COMPLICATION PROBABILITY OF THE LIVER IN THE
TREATMENT OF MESOTHELIOMA PATIENTS.
Frank Van den Heuvel, Yolande Lievens
UZ Gasthuisberg - KU Leuven
Dose volume histograms are a common tool to assess the value of a treatment plan
for various forms of radiation therapy treatment. The purpose of this work is to apply
a set of tools to analyze differential dose volume histograms by decomposing them
into physically and clinically meaningful normal distributions.
One of the advantages of such a decomposition technique is that it allows to estimate
the normal tissue complication probability (NTCP) in inhomogeneously irradiated
organs in a more refined manner. Indeed, radiobiological data can be taken into
account for every decomposed entity, and the NTCP can be calculated automatically
starting from a differential dose volume histogram.
In this study the NTCP for the liver was calculated for 11 patients undergoing IMRT
treatment for Mesothelioma. The treatment dose was set at 50.4 Gy in fractions of
1.8 Gy.
The dose volume histograms were obtained from an inverse planning system using a
dynamic IMRT implementation (Eclipse, Varian, Milpitas, CA).
The calculations were performed as described by Mc Ginn et al. using an error
function based expression for NTCP. In the latter article the treatment dose was
tailored so that the NTCP to the liver did not increase 10%.
The NTCP ranged from 0.4% to 20%. It was clear and not surprising that the NTCP
for patients with right lung involvement was significantly higher. In our conclusion it
are these patients specifically who will receive benefit from an IMRT based treatment
while the other patients can be treated conventionally with the same complication
probability.
Keywords : treatment planning, dose volume histogram, radiobiology
02 – A VARIABLE TG 43 APPROACH TO BRACHYTHERAPY USING
192IR WIRE IMPLANTS.
Frank Van den Heuvel, Marisol De Brabandere
UZ Gasthuisberg - KU Leuven
Currently, Ir-wire applications are still being calculated with a methodology developed
in the early 80s. While clinical data support the adequacy of this approach. There are
still questions pertaining the way dose is prescribed as well as the way source
strength is provided by various standardized laboratories.
The current standard of practice requires source strength to be defined as the Air
Kerma strength. The still prevalent Paris technique still utilizes apparant activity.
We propose to alter the characterization of the Ir-wire source to closely follow the
AAPM Task Group 43 (TG43) description of brachytherapy sources. This allows the
Ir-wire to be easily incorporated in modern brachytherapy systems. In contrast to
other sources typefied by TG43 the geometrical length of the Ir-wire is not a fixed
constant, which influences the geometrical parameters of the dose distribution. We
therefore initiate TG43 calculations for different lengths (e.g. 1 to 14 cm long wires).
A general purpose monte carlo simulation program (MCNPX) is used to generate a
cylindrically symmetric dose distribution in: 1) wet Air, 2) Water, and 3) Polystyreen.
An additional dose point is added to serve as a reference at the center of the wire in
the long direction, 1cm away from the source center in the away direction.
The following characteristics are then scored: The dose rate constant:
  D r0 ,  0  S k the radial function g r  and the geometric function F r,  . All
functions are parametrized (L) with respect to length. Note that the source strength
S k needs to be determined from the MC-calculations.
This paper aims to:

Determine a generalized form of the TG43 formalism with a parameter L,
fitted to the values obtained above.

Estimate the differences of using partial lengths (i.e. a 6 cm wire described by
the sum of two 3 cm wires)
Keywords : brachytherapy, TG43, Ir
03 – RADIOTHERAPY SOFTWARE FOR DEPARTMENT MANAGEMENT
AND ORGANISATION
Maxime Coevoet, Laurette Renard, Françoise Vanneste, Jean-Marc Denis,
Samira Chellouki, Pascale Henderickx, Ayhan Findik.
Clinique Universitaire St-Luc (UCL)
The “Radiotherapy Intranet” is a software developed to improve organisation in a
radiotherapy department and to have a transfer of information between the different
teams. It gives a global overview of the work to do.
We have made a real study of what is the job of everyone in a radiotherapy
department. The result is a checklist of all the things to do. From the consultation to
the first treatment day of the patient, trough the first simulation, planning, realisation
of cerrobend blocks, … Each checklist is customised for each different patient
planning.
Practically, the main page of the Intranet is the list of the different patients names in
different background colours. Each colour belongs to one trade (red for physicists,
blue for nurse, … ). When the stage is finished the background colour of the patient
name will change to the new one. And someone else can deal with the continuation
of the treatment preparation.
Radiotherapy is a teamwork. The Intranet can't work if every involved uses it
correctly. But now, after one year of use, we can say that it became difficult to work
without this huge information database.
Moreover, we are able to get some statistics on the time needed for a particular
activity, on improvement of organisation, time lost in the procedures, number of
patients, ...
Technically, the Intranet works like a website. The database is running on a local web
server, each computer can access to it via a web browser like Internet Explorer,
Mozilla Firefox, Epiphany, ... as well under Windows, Linux or Macintosh, and, the
most advantage for a radiotherapy department, without any installation on each
client.
Is it is an internal website, confidentiality is 100% guaranteed.
Keywords : radiotherapy, software, management
04 – TENTATIVE OF COMPARISON BETWEEN STEP AND SHOOT AND
DYNAMIC (SLIDING WINDOW AND HELICAL TOMOGRAPHY) IMRT FOR
HEAD AND NECK CANCER USING A SIB APPROACH
Milàn Tomsej (1), V. Tondeur (1), Pascal Fenoglietto (2), Nathalie de Patoul (1),
Norbert Aillères (2), Vincent Grégoire (1)
(1) Université Catholique de Louvain (U.C.L.), Cliniques universitaires Saint-Luc
Service de radiothérapie oncologique, 10, avenue Hippocrate, B-1200-Bruxelles,
Belgique
(2) Centre Val d’Aurelles- Service de radiothérapie, Montpellier- France
Intensity modulated radiation therapy (IMRT) plays an important role in the treatment
of patients with head and neck cancer. As the highly conformal dose distribution that
is achievable with IMRT makes it possible to envisage an increase in physical dose
while still maintaining the dose to the OAR at a reasonable level, several options
could be considered to attain this objective.
This has been possible thanks to the rising of quality of inverse treatment planning
systems employing very powerful algorithms, focused either on optimisation either on
dose calculations. Optimal fluences are found for each beam by the input of dose
volume constraints and will generate different shapes of beams using wether a
dynamic or a static collimation. Linacs today are capable to treat in IMRT patients
using the “step and shoot” or the dynamic delivery or both.
This study tries to compare both delivery techniques (step and shoot and dynamic),
with ELEKTA (step and shoot only possible) and VARIAN (step and shoot and
dynamic) linacs using the same inverse planning system, ECLIPSE (VARIAN), in
which both kinds of beam delivery exist, and is the result of a collaboration of 2 sites
owning original equipment, i.e. Centre Val d’Aurelles in Montpellier, France
(ECLIPSE and VARIAN linacs) and St-Luc University Hospital, U.C.L. in Brussels,
Belgium (ECLIPSE and ELEKTA linacs).
Of course, due to the integration of a new Hi-ART (Tomotherapy Inc.) in Brussels, its
dynamic helical delivery mode has also been tested and studied.
Same patients have been exchanged in terms of images and volumes and were
chosen to belong to same category,say moderately advanced (T2N0, T2N1, T3N0)
squamous cell carcinomas (SCC) of the oropharynx, larynx and hypopharynx, treated
using a simultaneous integrated boost approach. Obviously, same dose volume
objectives (and/or same optimal fluence files) have been fixed as well as the
maximum iterations and of course the geometrical parameters and beam energy.
Treatment plans are performed according to such protocol, and analysis has been
realized reporting parameters derived from dose volume histograms and will be
discussed here.
Keywords : IMRT, SIB, delivery mode, tomotherapy
05 – VALIDATION OF MRI TO CT SIMILARITY MEASURE VOXEL-BASED
FRAMELESS REGISTRATION FOR FRAME-BASED RADIOSURGERY
TREATMENT PLAN
F.De Smedt(1,2), D.Wikler(1), M. Op de beeck(1), N.Massager(1),
D.Deviendt(1,2), S. Simon(1,2), M. Levivier(1)
(1) Hôpital Erasme, Service de Neurochirurgie, Centre Gamma Knife
(2) Institut Bordet, Service de Radiophysique
INTRODUCTION – Gamma Knife radiosurgery uses a frame-based approach for
multimodality medical image registration and treatment application to provide
superior accuracy and reproducibility. The recent advent of frameless techniques for
the registration of medical images and their accuracy assesment foster the use of
multimodal information in radiotherapy and radiosurgery treatment planning systems.
Frame-based procedures could benefit from frameless registration in using
diagnostic MRI for the treatment planning along with frame-based CT hence reducing
organization and timing constraints. Our study aims at assessing the clinical validity
of such an approach for Gamma Knife interventions.
MATERIAL AND METHODS – 50 radiosurgery treatment planning (RTP) data from
Gamma Knife patients were retrospectively included in our study. For each RTP, the
frame-based MRI pulse sequences used in the treatment plan were registered using
the similarity measure voxel-based registration provided by the new release of the
LGP4C Gamma Knife RTP system (Elekta, Sweden). Registration parameters were
extracted from LGP4C for frameless and frame-based algorithms as applied to MRI
pulse sequences (Tmri-ct, Tmri-frame) and CT (Tct-frame) series. From these
matrices, the error geometric transform generated by the frameless technique as
opposed to the frame-based registration was derived: Terror = Tct-frame * Tmri-ct *
inv(Tmri-frame). This error transform was then applied to the treatment prescribed
isodose volume (PIV) leaving the target volume (TV) unchanged to simulate errors
due to the frameless registration on the placement of isocenters. A modified
conformance ratio (TVpiv*PIV/TV where TVpiv is the target volume fraction covered
by PIV) was computed and compared to the original conformance ratio to assess the
validity of the frameless registration.
CONCLUSION – Results and their classification according to treatment indication
and volume suggest that registration of frameless MRI to frame-based CT can be
achieved. Accuracy and reproducibility is dependent on specific guidelines for the
MRI registration strategy. MRI high resolution 3D gradient echo T1 can be registered
to CT with accurate and reproducible results when initial conditions are not too far
from the final solution while MRI high resolution selective partial inversion recovery
T2 SPIR cannot be registered accurately to CT.
Keywords : registration, radiosurgery, Gamma Knife
06 – ACCEPTANCE AND COMMISSIONING OF A SYSTEM FOR HELICAL
TOMOTHERAPY : WORKING IN NON-REFERENCE CONDITIONS.
Koen Tournel, Dirk Verellen, Nadine Linthout
Radiotherapy Department ,Oncoligisch Centrum, Academisch Ziekenhuis VUB
Earlier this year a commercial system for delivering intensity modulated radiation
therapy using a helical, rotational delivery (TomoTherapy, TomoTherapy inc.,
Madison US) was installed at the AZVUB. The system consists of a 6MV linac
mounted on a slipring, and the center of rotation is located at a source-axis distance
(SAD) of 85cm. The beam intensity is modulated using a binary MLC consisting of 2
banks of 32 leaves. Possible field sizes are resp. 1, 2.5 and 5 cm by 40cm.
Treatments are performed by continuous movement of the couch while the linac is
generating radiation.
The acceptance and commisioning procedure of the system that was provided by the
company only consisted of a black-box technique that verified the entire treatment
chain as a whole and did not involve intensive testing of different parts as described
in NCS report 9 , especially regarding the absolute dosimetry of the system.
Because of the particular design (SAD 85cm, no 10x10 field, no flattening filter, no
monitor unit specification) it is not possible to apply the standard dosimetry protocols
to this system. To be able to describe and check standard and action levels for the
different dosimetric parameters of the system a check using an adapted version of
the TG51 protocol was applied and-cross linked to the HPA-based calibration of the
other accelerators in our institution. Since the ionisation chambers to which the
system is customized (Exradin A1SL, Standard Imaging, US) is not widely spread in
Europe the chambers were recalibrated in the national standard laboratory and
cross-linked to the standard dosimetric protocol for photon beams (HPA) in our
department. Since only phantom checks using a homogeneous phantom and
company supplied-ionisation chambers were possible on the system a procedure
was developed to be able to check dose in a RANDO head-and-neack phantom
using TLD and film.
The results of the recalibration of the company-supplied chambers show a
discrepancy of 2-3% relative to the value supplied by a US-standard laboratory. After
this correction the absolute dosimetry was within 0.5% of the value generated by the
other machines in the department and using another protocol. The procedure of
calibration of film and TLD-dosimeters proved valid within 2-3%, allowing the use in
relative dosimetry.
Keywords : helical tomotherapy, dosimetry, commissioning
07 – DYNAMIC ARC THERAPY: FEASIBILITY FOR PROSTATE CANCER
IRRADIATION.
Harzee L., Dessy F., Merlo P., Hoornaert M.T.
Hospital of Jolimont, Haine-Saint-Paul, Belgium
PURPOSE – To study the feasibility of using dynamic arc therapy to treat patients
having a prostate cancer for the boost or the total treatment (good prognostic)
MATERIALS AND METHODS – Dynamic arc therapy can be delivered by our Varian
linear accelerator Clinac 2100C/D (120 leafs MLC). In this study, two Treatment
Planning Systems are used : Eclipse, version 6.5.7.2.24 from Varian and Brainscan
version 5.3. from Brainlab. Both TPS use a pencil beam algorithm. Treatment plans
using 6 MV photon beams are generated on both TPS with identical field setup. The
treatment parameters are then exported to the treatment machine through the Varis
network and delivered to the body phantom Omnipro IMRT from Scanditronix. Kodak
EDR2 films are placed between each slice. They are digitized with a VIDAR VXR12
Plus associated with the software Omnipro, version 1.4.1.0 from Scanditronix. With
this method, 3 comparisons are possible: Eclipse-Brainscan to compare calculated
treatment plans, Eclipse-Measurements & Brainscan-Measurements to evaluate the
agreement between each calculated dose distribution and the measurement, using
the gamma factor (= 3% ,  = 3 mm). In addition, the dose at the isocenter is
determined with a 0.6cc ionisation chamber and relative punctual measurements are
made at the isocenter and in the middle of the rectum with a PinPoint chamber
0.015cc from PTW. In the second part of this study, using Eclipse, we try to find the
best field configuration (1 or 2 dynamics arcs) to treat this organ and we compare our
methods and results with the literature.
RESULTS – The results from the comparison between calculated and measured
dose ditribution are discussed. The limitations of both TPS for this type of treatment
are presented, as well as the best configuration obtained with Eclipse.
CONCLUSION – These preliminary results, together with an easy and quick
treatment delivery, encourage us to go on with comparison with our actual
techniques before implementing this technique in clinical routine.
Keywords : dynamic arc, prostate, gamma index
08 – THE EVALUATION OF TWO DIFFERENT IMRT TREATMENT
DELIVERY SYSTEMS, HELICAL TOMOTHERAPY AND STEP ‘N SHOOT
IMRT : AN ALANINE DOSIMETRIC VERIFICATION.
Michael Duchateau, Dirk Verellen, Bob Schaeken
Xios Hogeschool Diepenbeek, AZ-VUB Jette, ZNA-Middelheim
BACKGROUND AND PURPOSE – Verification of IMRT delivered dose is challenging
(small fields, high dose gradients). This study evaluates absolute dose administered
with two different techniques; helical tomotherapy (AZ-VUB, Jette, Brussels) and step
and shoot IMRT treatment delivery (ZNA-Middelheim, Antwerp), using alanine
dosimetry.
METHODS AND MATERIALS – Tomotherapy(TM) IMRT is delivered using the
HiART system and the step and shoot IMRT is delivered with a SLi linac
(Elekta(TM)). The Elekta SLi uses the Pinnacle(TM) treatment planning system and
the Tomotherapy planning station uses a specific developed treatment planning
system. Both systems are based on the superposition/convolution principle to
perform the dose calculations. An anthropomorphic phantom is used for the
measurements. Several different tumour contours, located in the head and neck
region, will be copied or drawn on to the phantom. Dosimetry with alanine, using
electron spin resonance (ESR), will be used to evaluate the effectively delivered dose
of these IMRT delivery techniques. The alanine dosimeters are positioned in the PTV
region and organs at risk.
RESULTS AND CONCLUSION – The “copy-to-phantom” methodology is not suitable
for a correct comparison because of the software differences in the quality assurance
module of both planning systems. Therefore a “phantom-is-patient” methodology will
be used for comparison and dosimetric evaluation. Dosimetric verification using film,
TLD and alanine will be shown. Preliminary results obtained with alanine on the
elekta Sli for a brain lesion show that 2 out of 10 measured points were
overestimated with more than 5%. Measurements for the head and neck lesion show
more than 5% overestimation in 3 out of 18 measured points and 3 out of 18 points
have an underestimation of more than 5%. Performed TLD measurements on the
tomotherapy HiArt system did not lead to a firm conclusion because of the TLD
positioning in high gradient zones. Film measurements on the tomotherapy HiArt
system were evaluated using a gamma technique with following parameters ; search
distance : 3 mm, DTA tolerance : 3 mm and dose tolerance : 0,075 Gy (3% of 2,5
Gy).
Keywords : tomotherapy, step and shoot IMRT, alanine dosimetry
09 – CHANGE MANAGEMENT IN RT
Tom Depuydt, Yolande Lievens
University Hospital Leuven Gasthuisberg
The turn of the century brought the large-scale break through of 3D conformal and
intensity modulated radiation therapy techniques in the clinical setting. During the
90’s, new technologies were introduced for high-resolution dose delivery. These
delivery techniques have nearly reached maturity. However to fully use the power of
high-conformal dose delivery up to its potential, the main focus of innovation today is
mainly on target volume definition and localization. Definition and localization of the
target volume requires advanced image acquisition technology for visualization of
anatomy. Some of the required technology and know-how is available in the
radiology industry. An integration process of this technology into radiation therapy
was commenced and input from technological innovation in radiology became one of
the driving forces for innovation in radiation therapy. As a consequence, these new
devices found there way to application in radiation therapy much faster then during
the previous decades. The last few years, the speed of technological evolution in
radiation therapy increased and this will probably continue in the future. Radiation
therapy innovating at higher speed influences the process of replacing equipment.
Beside the installation of new equipment there seems to be an almost continuous
process of upgrading hardware and software in our departments. To handle this
continuous process of change one has to incorporate change into the daily routine.
Radiation therapy being a complex working environment with its different disciplines
working together is a particularly difficult environment to implement new approaches
to change.
The radiation therapy department of the University Hospital Gasthuisberg Leuven is
at the moment going through the motions of changing about 75% of its RT
equipment. This talk is about the impact of such a project on a radiation therapy
department with a heavy workload of treating 2400 patients every year on 5
treatment units.
Keywords : change management, innovation, IGRT
10 – MEASURING THE SKIN DOSE OF PATIENTS IN INTERVENTIONAL
RADIOLOGY: STANDARDIZATION OF THE CALIBRATION OF THE
THERMOLUMINESCENT DOSIMETERS
N. Jans (1,2), L. Struelens (2), M. Breugelmans(1)
(1) XIOS Hogeschool Limburg, Master in Industrial Siences, Nuclear Technology,
Diepenbeek
(2) SCK-CEN, Radiation Protection division, Mol
BACKGROUND AND PURPOSE – Interventional radiology deals with radiological
procedures where problems of the vascular structure are treated. These
examinations can be very complex. The doses for the patients can be very high and
next to stochastic effects, also deterministic effects have to be taken into account.
The most important tissue of concern, relating to these latter effects is the skin.
Therefore it is important that the skin dose of the patient could be measured as
accurately as possible. The most direct measurement of this skin dose is done with
thermoluminescent dosimeters (TLDs). The calibration of TLDs is crucial in the
measurement process, but no standardization exists in the way these calibrations
should be performed.
METHOD AND MATERIALS – It is important that TLDs are calibrated in a situation
which corresponds to the practice where they are used. By this, we mean the energy
used, the way they are packed and treated. If different circomstances are used for
the calibration, this might have an influence on the response of the TLD and doses
may be calculated less accurate. Several experiments are carried out to investigate
the influence of different exposure conditions on the response of TLDs. 2 types of
TLDs were used: LiF:Mg,Ti (TLD100, USA) and LiF:Mg,Cu,P (MCP-N, Poland). TLDs
were packed into different materials: in plastic bags (whether or not sewed on
different kind of tissues), in a plexi standard, behind 2 mm of PMMA, etc. Some of
the TLDs were also used without packing. The experiments can be categorised in 4
exposure classes: on a waterphantom, on the Rando-Alderson phantom, on wood
and free-in-air. Different energies were taken into account: narrow and broad x-ray
reference ISO spectra, a Co- and a Cs-source.
RESULTS AND DISCUSSION – For every exposure class, response curves in
function of energy were plotted for the different packing methods. They are
calculated as well in reference to air Kerma as to Hp(0.07). As expected, we clearly
found differences in TLD response for the different exposure conditions. All these
curves are thoroughly evaluated, in the clinical energy field of interest, to conclude,
among others, which experiment combination is the least depending on energy,
which combination of exposure condition (energy and phantom) and packing method
has similar responses as those for the combinations used most frequently in practice,
how large are the differences, etc.
Keywords : skin dose, TLD, calibration
11 – THE INFLUENCE OF THE PATIENT SIZE ON THE CONVERSION
COEFFICIENTS FOR THE CALCULATION OF THE EFFECTIVE DOSE
Katrien Leysen (1,2), Lara Struelens (2) and Herwig Janssens (1)
(1) Xios Hogeschool Limburg, master in industrial sciences, nuclear technology,
Diepenbeek
(2) SCK-CEN, radiation protection division, Mol
BACKGROUND AND PURPOSE – According to the legislation, the registration of
patient doses in conventional radiology is necessary. In view of optimisation of
patient doses, also the effective dose might be a useful tool. Conversion coefficients
are used to calculate this effective dose, with the aid of easily measurable quantities
like dose-area product (DAP) or entrance surface dose (ESD). Mostly, those
conversion coefficients are used, which are calculated for standard phantoms
(length: 179cm and weight: 73,5kg). Because every patient is different, however,
inaccuracies are introduced using these standard conversion coefficients. This study
evaluates the difference in effective dose and organ dose conversion coefficients
between a standard model and two thicker phantoms (one with length: 179cm and
weight: 83,5kg and the other with length: 179cm and weight: 93,6kg).
METHODS AND MATERIALS –The simulation of an androgynous phantom is made
by the software BODYBUILDER. The advantage of this software is the possibility to
add an extra layer of fat. The calculation of the effective dose and a theoretical
DAP/ESD-value is delivered using the computer code MCNP4B. This is done for
different examinations (abdomen AP and thorax PA) and different clinical spectra.
For the mentioned procedures above, DAP and ESD are measured on different
patients in some radiology departments. The technical parameters used (tube
voltage and tube current), aswell as the patient dimensions, were also registered.
RESULTS AND CONCLUSION – The MCNP calculations lead to 3 tables of
conversion coefficients, one standard table and two tables for thicker phantoms. The
patient data in the hospitals will tell us how the procedure is performed in function of
patient thickness. These results will show us the degree of inaccuracy that is made
by using the standard conversion coefficients for effective dose calculations,
according to patient thickness. Also the influence of possible changes of tube voltage
for thicker patients in practice is investigated.
Keywords : conversion coefficient, effective dose, patient thickness
12 – ACCURACY OF PATIENT DOSE CALCULATION FOR LUNG-IMRT: A
COMPARISON OF MONTE CARLO, CONVOLUTION/SUPERPOSITION
AND PENCIL BEAM COMPUTATIONS
Barbara Vanderstraeten (1,2), Nick Reynaert (1), Leen Paelinck (2), Indira
Madani (2), Werner De Gersem (2), Carlos De Wagter (2), Wilfried De Neve (2)
and Hubert Thierens (1)
(1) Ghent University, Department of Medical Physics
(2) Ghent University Hospital, Department of Radiotherapy
We compared full Monte Carlo calculations (performed by our Monte Carlo Dose
Engine MCDE) with two different commercial convolution/superposition
implementations (Pinnacle and Helax-TMS’s collapsed cone model Helax-CC) and
one pencil beam algorithm (Helax-TMS’s pencil beam model Helax-PB) by
computing dose distributions within 10 lung-IMRT patients. Treatment plans were
created for two photon beam qualities (6 and 18 MV).
For each dose calculation algorithm, patient and beam quality the following dosevolume values were reported: minimal, median and maximal dose (Dmin, D50 and
Dmax) for the gross tumor and planning target volumes (GTV and PTV); the volume
of the lungs (excluding the GTV) receiving at least 20 and 30Gy (V20 and V30) and
the mean lung dose; the 33rd percentile dose (D33) and Dmax delivered to the heart
and to the expanded oesophagus; and Dmax for the expanded spinal cord.
Statistical analysis was performed by means of 1-way analysis of variance (ANOVA)
for repeated measurements and pairwise comparison of means according to Tukey’s
method. Pinnacle showed an excellent correspondence with MCDE within the target
structures, whereas the best correspondence for the organs at risk (OARs) was
found between Helax-CC and MCDE. Results from Helax-PB were unsatisfying both
for targets and OARs.
Additionally, individual patient results were analyzed. Deviations larger than 5% were
found in 1 patient for the comparison of MCDE and Helax-CC within the target
structures, while for Pinnacle all dose differences were below 5%. Both for Pinnacle
and Helax-CC, substantial deviations from MCDE were found within the OARs.
Pinnacle showed deviations in dose-volume values within the lungs larger than 5%
for two (6 MV) and six patients (18 MV), respectively.
In conclusion, not one pair of the dose calculation algorithms we investigated could
provide results that were consistent (within 5%) for all 10 patients for the set of
clinically relevant dose-volume indices studied. As the results from both
convolution/superposition algorithms differed significantly, care should be taken when
evaluating treatment plans, since the choice of dose calculation algorithm may
influence clinical results. Full Monte Carlo provides a great benchmarking tool for
evaluating the performance of other algorithms for patient dose computations.
Keywords : dose computation, lung IMRT, Monte Carlo
13 – COMMISSIONING OF A NEW VARIAN TPS ALGORITHM (AAA)
CONSIDERING STEP AND SHOOT IMRT TREATMENTS : DOSIMETRY
COMPARISON WITH MEASUREMENTS AND MONTE CARLO
(BEAMNRC) CALCULATIONS USING A ELEKTA SL25 LINAC
Edmond Sterpin, Milàn Tomsej, Nick Reynaert and Stefaan Vynckier
Université Catholique de Louvain - Cliniques universitaires Saint-Luc. Radiotherapy
Department
AAA (Anisotropic Analytical Algorithm) is a pencil beam convolution/superposition
algorithm involving Monte Carlo derived data for primary photons and for scattered
photons and electrons. Moreover, the dose deposition is calculated by using six
exponential curves. Inhomogeneities are taken into account anisotropically thanks to
photons scatter kernels in multiple lateral directions.
Monte Carlo calculation uses instead physical cross sections to simulate the particles
transport depending on the medium. We expect therefore more accurate results for
heterogeneities with Monte Carlo calculations than analytical algorithms.
Our Monte Carlo model, using BEAMnrc software and the modeling of the ELEKTA
linear accelerator were validated in standard conditions achieving measurements’
and calculations’ comparisons in a water phantom. The model has been tuned by
adapting electron energy and the opening of the jaws and the leaves.
AAA has not been validated yet for ELEKTA linear accelerators. We use our model
and measurements to study the accuracy of the results given by AAA firstly in a
water phantom for standard and IMRT fields. Secondly, further comparisons will be
performed considering a phantom containing inhomogeneities. Finally, we compare
AAA with Monte Carlo calculations in a CT scan for a complete IMRT treatment
considering a head and neck tumor case.
Keywords : Monte Carlo, IMRT, AAA
14 – TANDEM METHOD FOR CALIBRATING DOSE-AREA PRODUCT
METERS
Paula Pöyry, Tuomo Komppa and Antti Kosunen
STUK - Radiation and Nuclear Safety Authority
Dose-area product (DAP) meters are used in diagnostic radiology to evaluate
radiation exposure to patient. DAP meters are transmission-type plane ionization
chambers providing a reading proportional to the surface integral of absorbed dose to
air over the effective area of the chamber. To provide accurate results for
comparative studies, the DAP meter should be calibrated properly.
A conventional way of calibration of DAP meter of an X-ray equipment, the field
meter, is to approximate the surface integral by the product of the field area and the
dose measured in the centre of the field, but the accuracy of this method may be
inadequate for the most demanding purposes [1]. The accuracy of DAP
measurements can be improved by calibrating the DAP meter relative to the surface
integral of the absorbed dose. A practical way of doing this at a hospital is to use a
tandem method where another DAP meter is used as reference instrument, the
reference meter [2]. In the most common variation of tandem method, the field meter
and the reference meter are used simultaneously in the X-ray beam. The reference
meter has to bee calibrated relative to the incident beam, thus providing for the field
meter a calibration relative to the beam incident on the patient.
The response of a typical DAP meter depends significantly on the energy distribution
of the X-ray beam. The half-value layer (HVL), generally used to specify the radiation
quality, is not adequate alone to determine the response of a DAP meter properly.
Even at a constant HVL value, the calibration coefficient of a DAP meter can typically
vary about 10 % in X-ray beams with different spectra. For DAP calibration, the
radiation quality can be specified by the tube voltage and beam filtration, and an
adequate number of clinical radiation qualities must be used to achieve reasonable
accuracy.
The tandem method for calibrating field DAP meters is described in this work. The
total relative uncertainty of the calibration coefficient (in good practical conditions)
was estimated to be ±6 % with a confidence level of 95 %. The work is part of a
project to establish a calibration service for DAP meters in Finland and to provide
dosimetric guidance for users.
[1] J. P. Larsson, J. Persliden and G. Alm Carlsson, Phys. Med. Biol. 43 (1998) 599.
[2] P. Pöyry, T. Komppa, A. Kosunen, Proceedings of the XXXIX annual conference
of the Finnish physical society (2005) 247.
Keywords : DAP meter, dose-area product, calibration, tandem method
15 – COMPARISON OF EQUIPMENT PERFORMANCE FOR X-RAY
SYSTEMS IN INTERVENTIONAL CARDIOLOGY
Octavian Dragusin, Hilde Bosmans, Kristien Smans, Peggy Vander Henst,
Walter Desmet
Catholic University of Leuven, Gasthuisberg Hospital, Department of Radiology
Interventional cardiology procedures are among the most common group of
interventional procedures performed in Belgium. The EU Medical Exposure Directive
97/43/Euratom and national legislation consider Catheterization Laboratory as an
area of special concern regarding the dose levels.
The paper presents the results of performance tests of 16 interventional cardiac
systems (14 systems with image intensifier and 2 systems with flat panel detectors).
The main goal of this survey was to assess the requirements for equipment
standards and to address the imbalance between the advancing technology and
existing standards. Testing was performed in line with current national and
international guidelines.
The quality controls included the assessment of the performance of the X-ray
generator, the dose levels in fluoroscopy and digital acquisition modes and image
quality using a subjective method (the Leeds test object). All doses (the detector and
patient entrance dose rates) were measured under AEC (Automatic Exposure
Control) and for various scattering material.
Analysing the results showed 2 remarkable results:


for new and old systems, dose levels are manufacturer dependent.
Consequently, dose rates were found to be very different between systems, in
both the fluoroscopy and digital acquisition modes.
In terms of image quality, all systems were found to have a measured
threshold contrast of < 4% and no measurable improvement in image quality
was observed for the systems operating at higher dose levels
From these observations, we conclude 2 facts: it is needed to carefully verify working
conditions (including the pre programmed settings of the systems) and to check the
effectiveness of quality control protocol and test tools.
Keywords : interventional cardiology, quality assurance tests
16 – ACCURACY OF THE TPS CALCULATION OF MATCHED HALFBEAM DOSE DISTRIBUTIONS
S. Petillion, A. Swinnen, M. De Brabandere, F. Van den Heuvel
University Hospital Leuven
Department of Oncology and Experimental Radiotherapy
PURPOSE – For some clinical indications with large planning target volumes (PTVs),
the dose distributions are not satisfactory. To improve homogeneity, half-beam
blocked fields can be matched. The use of matched half-beams is possible thanks to
the advent of independent jaws in most clinical accelerators. This study checks
whether a commercial treatment planning system (TPS) is able to accurately
calculate the dose in the matching region.
METHODS – Four different isocentric irradiation conditions, each consisting of two
half-beams, are considered: matching using the upper and lower jaws, with and
without backing MLC-leaves. The total size of the two beams combined is 24 x 20
cm². An EDR2-film is positioned at a depth of 12cm in a polystyrene phantom, and
irradiated with 18MV photon beams, delivering 2Gy to the central axis at the film
position. Using a densitometer the measured dose distributions are determined, and
subsequently compared to the calculated dose distributions.
RESULTS – It is found that the calculated dose distributions are identical for all four
setups. The TPS predicts a small dose dip at the connection of the half-beams. In
contrast, the measured dose distributions are mutually different. It is shown that if the
matching is performed with the upper jaws, the experimental results are influenced
by the position of the MLC-leaves. If no MLC-backing is present, there is no dose
reduction observed in the matching region. For closed leaves under the jaw (e.g.
adjusted to PTV), the film measurement shows a larger dose dip than calculated. If
the shaping of the half-beams is performed with the lower jaws, the calculated and
the measured dose distributions are comparable, even in the matching region,
irrespective of the leaf positions.
CONCLUSIONS – Most likely, the discrepancy of the calculated and the measured
dose distributions is due to the inaccuracy in the beam modelling of the TPS, i.e. the
geometric penumbra and the tongue-and-groove shape of the MLC are not taken into
account.
Keywords : TPS calculation, matched half-beams, dose distribution
17 – FRAMEWORK FOR DIFFUSION TENSOR TRACTOGRAPHY
VALIDATION WITH AN ANATOMICALLY REALISTIC SOFTWARE
PHANTOM.
Steven Delputte, Rik Van de Walle, Ignace Lemahieu
Medisip (ELIS, UGent)
During the last decade, the development of MRI has led to a diversity of applications.
One of the more advanced ones is Diffusion Tensor MRI (DT-MRI). This technique
provides us with bulk averaged, microscopic anatomical information of fibrous tissues
such as skeletal muscles and brain white matter. Fiber tracking algorithms based on
diffusion tensor data are the first techniques capable of reconstructing the threedimensional architecture of the major white matter axonal bundles within the living
human brain. This information can help neuroscientists to better understand brain
effective connectivity and white matter diseases (Alzheimer’s disease, multiple
sclerosis,...) or can be used for better pre surgical planning.
Although diffusion tensor tractography is very promising, it’s current clinical use is
limited due to the lack of a golden standard for validation of this technique. Therefore
we propose a method to construct a noise-free synthetic diffusion tensor dataset,
based on a priori known tracts that resemble the true white matter anatomy. These
ground-truth fibers are found by performing Density Regularized Fiber Tractography
(DRFT, Delputte, Proc. ISMRM 2005) on in-vivo data. The diffusion weighted
volumes are smoothed with an anisotropic PDE-filter and we use a robust tensor
estimation technique to reject outliers from the final tensor fitting. This also removes
the need for cardiac gating. DRFT takes into account the architectural milieu of each
tract and results in 3D, non-spurious, axonal pathways with a pointwise estimate of
probability along each tract.
The construction of the synthetic tensor dataset is based on the framework of A.
Leemans (MRM 53, 2005). Instead of using fixed parameters for each fiber and for
each point along these fibers however, we use the pointwise architectural information
provided by the DRFT results. Thus, we use a varying width and steepness for the
saturated Gaussian fiber kernel, a probability estimate, and an actually measured
fractional anisotropy (FA) and apparent diffusion coefficient to make the dataset as
realistic as possible. Comparing the colour coded synthetic FA volume with the invivo one, we find indeed a good correspondence. After adding noise, we can run
different tractography algorithms on the datasets and quantitatively compare these
reconstructed fibers with the original ground-truth fibers. The framework was also
used to optimize internal and operator dependant tractography parameters.
Keywords : diffusion tensor tractography, quantitative validation, synthetic phantom
18 – PHANTOM DESIGN FOR DIFFUSION TENSOR MAGNETIC
RESONANCE IMAGING.
E.Fieremans (1), S.Delputte (1), Y.De Deene (2), Y.D'Asseler (1), E.Achten (3),
I.Lemahieu (1), R.Van de Walle (1)
(1) Ghent University, ELIS/MEDISIP, Sint-Pietersnieuwstraat 41, B-9000 Ghent,
Belgium.
(2) Ghent University Hospital, Department of Radiotherapy, De pintelaan , B-9000
Ghent, Belgium
(3) Ghent University Hospital, Department of Neuroradiology, De pintelaan , B-9000
Ghent, Belgium
Diffusion tensor magnetic resonance imaging (DT-MRI) is a non-invasive imaging
method that can measure the motion of water molecules in all directions. In tissues
containing a large number of fibres, like skeletal muscle and brain white matter, water
tends to diffuse mainly along the directions of those fibres. This way, diffusion tensor
imaging (DTI) has access to the organization in space of tissue micro structural
components. Moreover, fibre tracking algorithms make is possible to reconstruct the
three-dimensional structure of the major white matter tracts within the living human
brain.
To determine the accuracy and precision of DTI a validation is necessary which
requires a phantom with a well known structure. We’ve proposed a flexible hardware
phantom by using parallel fibres made of woven strands tightly held together by a
shrinking tube [1].In this abstract the possibility to develop a diffusion phantom for
validating DTI in brain white matter is investigated.
To test the possibility of an anthropomorphic head diffusion phantom, fascicles are
constructed in different diameters and geometries to imitate some of the major neural
fibre tracts: corticospinal tracts, optical tracts, corpus callosum (forceps maior and
minor) and fronto-occipital tracts. For this experiment phantom fascicles were placed
in gel of 1.25 % (w/w) agarose and 0.275 mM Gd-DTPA to obtain T1 and T2 values
similar to those of white matter and to reduce flow artefacts. These fascicles were
fixed in a container with a similar shape as the head that contains a hollow pipe
presenting the trachea.
To imitate brain white matter in DTI, a phantom material is necessary with similar
diffusion characteristics and proton density. Fascicles were made of different fibre
materials (silk filaments, silk yarn, Micro Dyneema®, dialysis fibers) - containing a
varying number of strands- and were compared for fractional anisotropy (FA) and
proton density (PD). Monte Carlo simulations were performed to study the diffusion
process within the phantom fascicles. The diameter of the used fibre filament and the
packing density pointed out to be the crucial factors to determine the behaviour of the
short- and long-time of the diffusion process within the phantom fascicles.
[1] E.Fieremans et Al., ISMRM Proceedings, 1301 (2005).
Keywords : phantom, diffusion, diffusion tensor imaging
19 – EXPLOITING THE CHARACTERISTICS OF A PROTOTYPE
ROTATING SLAT COLLIMATED SOLID STATE STRIP DETECTOR: DUAL
ISOTOPE IMAGING.
Roel Van Holen, Steven Staelens, Stefaan Vandenberghe, Yves D'Asseler and
Ignace Lemahieu
ELIS-MEDISIP, Ghent University (UGent), Ghent, Belgium
OBJECTIVES – Solid state detectors offer exquisite energy resolution compared to
scintillation detectors. A dual isotope 123I/99mTc subtraction study investigates if the
parathyroid (PT) localization with the prototype Solstice gamma camera is superior to
traditional gamma camera scintigraphy with a NaI detector crystal.
METHODS – A thyroid/parathyroid phantom was built from CT data. The thyroid
volume is 25ml and the two parathyroids have a volume of 0,2ml and 0,5ml. The
phantom was filled with 123I and 99mTc activities based on literature values of
thyroid and PT adenoma tracer uptake. The total activity (Technetium and Iodine) in
the phantom was about 6 MBq, based on a patient injection of 750 MBq. Using the
prototype Philips SOLSTICE camera (rotating slat collimator on a solid state strip
detector) we performed a 20 minutes acquisition. Using the same phantom setup, the
measurement was repeated on a classical gamma camera (AXIS, Philips).
For the Solstice data, photopeak windows of 10% were set around 140 keV for
99mTc and 159 keV for 123I before reconstruction to projection images using Monte
Carlo based MLEM. Projection data from the Axis simulation were binned in a 256 by
256 pixels matrix using a 15% energy window for 99mTc (140 keV) and a 10%
window for 123I (159 keV). Subtraction of the iodine image (accumulating only in the
thyroid) from the simultaneously acquired technetium image (accumulating in both
the thyroid and parathyroids) yielded the image of the PTs. After qualitative
evaluation, the images were scored on the basis of detectability of PT glands D=(sμ)/σ where s represents the image counts in a region of interest drawn in the PT and
μ and σ are the mean and standard deviation of the surrounding background activity
respectively.
RESULTS – Visual inspection already shows the Solstice image to be superior to the
Axis image. We can point out where the parathyroid glands are on the Solstice image
while on the axis image, the localization can’t be done this accurately. Quantitative
analysis of the images showed the detectability D in the Solstice imaging to be better
than the Axis imaging by a factor 1,37 for the bigger adenoma and by a factor 1,71
for the smaller gland.
CONCLUSIONS – Due to the superior energy resolution and sensitivity of the
Solstice gamma camera, the localization and detection of the PT glands using the
99mTc/123I subtraction technique is showed to be clinically more performant than
the classical gamma camera approach.
Keywords : energy resolution, dual-isotope imaging, parathyroid scintigraphy
20 – A REVIEW ON THREE DIMENSIONAL RADIATION GEL DOSIMETRY
Yves De Deene
Radiotherapy (P7), Ghent University Hospital, De Pintelaan 185, 9000 Gent
The first gel dosimeters date back to the 1950's when a radiation-induced colour
change in dyes imbedded in a gel were used to investigate doses [1]. In 1984, Fricke
gel dosimeters that could be read out using NMR relaxometry were proposed [2].
These gels were based on the conventional Fricke solution that was dispersed
throughout a gel matrix. In these Fricke gels the Fe2+ ions are converted to Fe3+
ions upon irradiation. The difference in paramagnetic properties of both ions results
in a change in the NMR spin-spin relaxation rate (R1) that is radiation dose related.
Due to predominantly diffusion-related limitations, alternative polymer gel dosimeters
were subsequently suggested [3]. In polymer gel dosimeters, monomers are
dispersed in a hydrogel matrix. The monomers undergo a polymerization reaction of
which the degree is a function of absorbed radiation dose.
The radiation-induced formation of polymer influences the NMR proton spin-spin
relaxation rate. First attempts to use polymer gel dosimeters to visualize the dose
distribution of a conformal radiation treatment showed that necessary care should be
given to the NMR imaging techniques [4].
As the polymerization reaction is inhibited by oxygen, all free oxygen has to be
removed from the gels. For many years this was achieved by perfusing the gel with
nitrogen. Recently, the use of anti-oxidants has been proposed to cancel out the
inhibition by oxygen [5]. Emphasis has been layed on the radiation properties of the
different gel dosimeters [6].
As well as MRI, other quantitative techniques for measuring dose distributions
include optical and x-ray CT, vibrational Raman spectroscopy and ultrasound. Many
of the recent developments in the field of radiation gel dosimetry are communicated
at the international DOSGEL conferences [6,7].
[1] Day M J and Stein G J 1950 Nature 166: 146-7.
[2] Gore J C, Kang Y S and Schulz R J 1984 Phys. Med. Biol. 29: 1189-97.
[3] Maryanski M J, Gore J C, Kennan R P and Schulz R J 1993 Magn. Reson.
Imaging 11: 253-8.
[4] De Deene Y, De Wagter C, Van Duyse B, Derycke S, Mersseman B, De Gersem
W, Voet T, Achten E and De Neve W 2000 Magn. Reson. Med. 43: 116-25.
[5] Fong P M, Keil D C, Does M D and Gore J C 2001 Phys. Med. Biol. 46: 3105-13.
[6] DOSGEL 2004 Proc. Third Int. Conf. on Radiotherapy Gel Dosimetry (Ghent,
Belgium) ed. De Deene Y and Baldock C.
[7] DOSGEL 2001 Proc. Second Int. Conf. on Radiotherapy Gel Dosimetry (Brisbane,
Australia) ed. Baldock C and De Deene Y.
Keywords : polymer gel dosimetry, 3D radiation dosimeters, quantitative NMR
21 – A SINGLE MEASUREMENT-BASED METHOD FOR ABSOLUTE
QUANTIFICATION OF METABOLITES BY PROTON MAGNETIC
RESONANCE SPECTROSCOPY
M.S. Özdemir(1), Y .De Deene(2), E.Achten(3), Y.De Asseler(1) , I.Lemahieu(1)
(1) Gent University,Department of Electronics and Information
Systems,Sintpietersnieuwstraat 41,9000 Gent, Belgium
(2) Department of Radiotherapy,Gent University Hospital, De Pintelaan 185, 9000
Gent, Belgium
(3) MR-Department (-1K12),Gent University Hospital, De Pintelaan 185, 9000 Gent
Belgium
Magnetic Resonance Spectroscopy (MRS) is a non-invasive technique capable of
directly measuring the chemicals within the body. Results are typically expressed as
ratios between metabolite signals which vary between malignant and benign tissue.
Therefore, malignancy can be characterized by utilizing the ratio change between
certain metabolites of interest. However, the ratio-based results may not be plausible
in many cases as the change in ratio may arise from the changes in the numerator as
well as the denominator. Thus, presenting the ratio-based results can be misleading
and absolute quantification of the metabolite concentration is preferable as it resolves
possible ambiguities resulting from ratios. For absolute quantification of metabolites,
a reference signal must be acquired which can be either an internal or external
standard. Internal references have the advantage that they are insensitive to
systematic errors associated with B1, Bo field inhomogeneities, flip angle and other
pulse sequence related factors. Creatine (cre), a metabolite in the brain spectrum, is
most commonly used as an internal reference. However, recent studies showed
substantial regional and pathology-related changes in Cre [1]. An alternative
reference is unsuppressed tissue water signal as brain water content is relatively
well known and pathology related changes are relatively small. Nevertheless, water
referencing may not be convenient in certain MRS studies since it may be
prohibitively time consuming to record both water suppressed and unsuppressed
spectra. However, we previously showed that a single measurement by which only
an unsuppressed water spectrum is acquired can be adequate to perform absolute
quantification [2]. By using Singular Value Decomposition (SVD), metabolite signals
can be extracted from the spectrum of unsuppressed water signal which is used as
an internal reference. In this study, a single measurement-based method to quantify
proton MR spectra is presented and tested by phantoms. The limitations of the
technique such as the signal loss in the extracted metabolite spectrum due to SVD
and gradient-induced frequency modulation side bands are discussed and possible
solutions to these shortcomings are given.
References
[1] Stockler et al, Pediatr Res.,1994; 36(3):409-413
[2] Ozdemir et al, 22 nd ESMRMB,2005,18,222-223
Keywords :magnetic resonance spectroscopy, absolute quantification, SVD
22 – AN ADAPTIVE OFF-LINE RADIATION SCHEME FOR TREATMENT
OF PROSTATE CANCER USING CONE-BEAM CT SCANS
Tonnis Nuver, Jasper Nijkamp, Rianne de Jong, Annemarie Lakeman, Monique
Smitsmans, Peter Remeijer, Marcel van Herk, and Joos Lebesque
Department of Radiation Oncology, The Netherlands Cancer Institute – Antoni van
Leeuwenhoek Hospital, Amsterdam, The Netherlands.
BACKGROUND – We have developed an adaptive scheme for treatment of prostate
cancer based on kV cone-beam CT images that are obtained in the first six treatment
days. With this scheme a better estimate of the average prostate position and the
average rectum shape are obtained, and the PTV margin can be reduced from 10 to
7 mm.
METHODS – The cone-beam CT scans obtained in the first 6 treatment days are
matched on the planning CT scan using the pelvic bones. The obtained translations
are used to correct the patient position. A Shrinking-Action-Level protocol is used for
this set-up correction. Automatic 3D grey value matching is then used to match the
prostates of the cone-beam CT scans to the prostate of the planning CT scan. Each
match yields three translations (T_LR, T_CC, T_AP) and three rotations (R_LR,
R_CC, R_AP). Mean values for these translations and rotations are calculated and
used to move the prostate of the planning CT scan to the average position. For the
six cone-beam CT scans the outer rectal wall is delineated. For each CT slice 50
equidistant points are placed on each of the rectum contours. The first of these 50
points is placed at the dorsal side of the rectal wall. Corresponding points can then
be distinguished for the rectums of the various scans. The coordinates of
corresponding points are averaged to obtain points on the surface of the average
rectal wall. At the end of the first treatment week a new IMRT treatment plan is
calculated, which is based on the average prostate and rectum, and a PTV margin of
7 mm. During treatment weekly cone-beam CT scans used in the set-up protocol are
also used to verify that the prostate is inside the PTV.
RESULTS – So far, 10 prostate cancer patients were successfully treated with our
adaptive treatment scheme. The PTV volume was on average 28% smaller than for
conventional treatment. For all verification scans the prostate was positioned inside
the PTV volume. The average dose received by the rectum reduced by 0.0-21.4%,
and the equivalent uniform dose (a=12) reduced by 0.5-4.1%.
CONCLUSIONS – This is the first routine clinical application of soft tissue image
guidance for the prostate using kV cone beam CT. It does not only correct for patient
set-up error, but organ motion as well, making the planning a more accurate
representation of the real treatment. Contrary to adaptive treatment schemes that
use implanted markers, our method is non-invasive and allows for calculation of an
average rectum.
Keywords : IGRT, prostate, kV cone-beam CT
23 – THREE DIMENSIONAL RADIATION DOSIMETRY IN LUNGEQUIVALENT REGIONS BY USE OF A RADIATION SENSITIVE GEL
FOAM: PRELIMINARY RESULTS
Yves De Deene
Radiotherapy (P7), Ghent University Hospital, De Pintelaan 185, 9000 Gent
A polymer hydrogel foam is proposed as a potential three-dimensional experimental
dosimeter for radiation treatment verification in low-density tissue such as the lung. A
gel foam is created by heavy mixing a radiation sensitive polymer gel mixture in an
anoxic atmosphere. The gel foam sets after the addition of a strong anti-oxidant
(tetrakis-hydroxy-phosphonium salt) and during slowly rotating the gel foam mixture.
The gel foam dosimeter is based on the dose dependent polymerization of vinyl
monomers that are confined in the gel. The degree of polymerization has a
measurable effect on proton NMR properties. Optical microscopy of the gel foam
dosimeter reveals good similarity with the microstructure of lung tissue. The mass
density of the gel foam dosimeter is in the order of 0.25 - 0.35 g/cc.
Both NMR spin-spin relaxation rate (R2) and magnetization transfer ratio (MTR) have
been used to visualize the dose distribution of the gel foam dosimeter. It is found that
MTR has significant advantages as compared to R2 for mapping the dose
distribution. Spin-spin relaxation dispersion has been observed making R2
dependent on the density of the gel dosimeter and on the echo time intervals in a
multiple spin-echo sequence.
A method to extract the electron density of the gel dosimeter by use of NMR proton
density images is also presented. The post-irradiation stability of the gel foam with
respect to the NMR properties has been investigated. The spin-spin relaxation
dispersion and NMR diffusion measurements can be used to extract information on
the gel foam microstructure in a non-invasive way. Computer models are proposed
that simulate the behavior of the water molecules in the gel foam dosimeter
describing restricted diffusion and relaxation dispersion that is attributed to the
mesoscopic magnetic field inhomogeneities caused by the nitrogen bubbles.
These preliminary results show that the gel foam dosimeter has significant potential
in the experimental verification of radiation treatment planning software that can be
applied to low-density structures. Research is underway to investigate how the
density and microstructure of the gel foam dosimeter can be tuned.
It also demonstrates the possibility of using quantitative NMR to obtain relevant
information (such as proton and electron density) that could be used as input to
radiation treatment plans.
Keywords : polymer gel dosimetry, low-density dosimeters, quantitative NMR
24 – A NEW COMPUTER CODE MEDICUS FOR DETERMINATION OF
CROSS SECTIONS OF NUCLEAR REACTIONS USED FOR THE MEDICAL
RADIOISOTOPE PRODUCTION
A. I. Ibishi, and A. Hermanne
Vrije Universiteit Brussel, VUCY, Laarbeeklaan 103, 1090 Brussels, Belgium
The new computer code MEDICUS has been developed in MATLAB 6.5,
Mathematica 5, and Lahey Fortran 95 programming languages. Especially are
calculated cross sections of nuclear reactions, used for production of medical
radioisotopes, that are important in cancer radiotherapy, such as: flourine 18 (18F),
phosphorus 32 (32P), cobalt 60 (60Co), copper 67 (67Cu), strontium 89 (89Sr),
yttrium 90 (90Y), strontium 92 (92Sr), indium 111 (111In), iodine 125 (125I), iodine
131 (131I), cesium 137 (137Cs), samarium 152 (152Sm), samarium 153 (153Sm),
lutetium 177 (177Lu), rhenium 186 (186Re), rhenium 188 (188Re), and iridium 192
(192Ir). The nuclear cluster structure of these elements has been analyzed [1].
Suitable radioisotopes for radioimmunotherapy are investigated [2]. The manynucleon equations are solved in the framework of the EMBNCM theory, DWBA,
Direct Nuclear Cluster Reactions, Pre-equilibrium Reactions, Optical Model, and
Exciton Model with Cluster Emission [1]. Excitation energies up to 200 MeV, and
incorporate several other improvements in calculation approaches. Our calculations
shows dependence between the nuclear cluster structures of radioisotopes such as:
32P, 60Co, 67Cu, 90Y, 92Sr, 131I, 137Cs, and beta radiation energy, chemical
stabilities, accumulation properties, radioisotope half-life, and destroyed tumour cells.
Possibilities for injection of radioisotopes such as: 67Cu, 90Y, 177Lu, 186Re, and
188Re, with monoclonal antibodies into a cancer patient [2], in vivo behaviour of
labeled compounds to the specific organ, and effects of different nuclear cluster
structures of radionuclides, to catalyze binding, and destruction of tumours by
antibodies, are investigated. Code MEDICUS based on EMBNCM, gives better fit to
the data, than actually used codes. We have taken into account reaction
mechanisms [1]. Our results are important for the development of the new anticancer vaccine.
[1]. A.I.Ibishi, New Method for Calculation of Nuclear Cluster Structure of Nuclei, AIP
Conf. Proc. 768, 395, Melville, New York, USA, 2005.
http://link.aip.org/link/?APCPCS/768/395/1
[2]. Wilder et al, Radioimmunotherapy: Recent results and future directions,
J. Clin. Oncol. Vol 14, 1383 - 1400, 1996.
Keywords : MEDICUS, cross-sections, medical radioisotopes.
25 – MINIMIZING THE TIME SPEND ON PATIENT SPECIFIC QA FOR
IMRT
Jan Verstraete, Bianca Vanstraelen, Tom Depuydt, Frank Van den Heuvel
UZ Gasthuisberg Leuven
Although IMRT improves the conformity of the treatment, it requires more time for
quality assurance (QA). Essentially QA for IMRT has 3 main levels: QA of the
treatment planning system, QA of the treatment equipment and patient specific QA.
Some centers focus primarily on the first 2 and tend to rely on the stability of there
equipment. Limited effort is put in the patient specific QA and some centers no longer
execute these checks. At UZ Leuven we are following another approach and kept
doing patient specific QA while minimizing the time spend on this procedure. This is
necessary with about 100 IMRT patients or around 530 fields per year.Until
September 2005 we focused on the use of absolute film dosimetry to perform routine
patient specific QA. In this methodology, a hybrid plan is created for every treatment
field in a RW3 phantom and the dose is calculated at a depth of 3 cm. The fields are
delivered on the treatment machine to the phantom (one film per field) using the
actual treatment plan at a zero gantry angle. To rule out differences in film
development conditions, all the films are developed at the same time and digitized
using a VIDAR 12-bit scanner. With two supplementary films, a specific sensitometric
curve per patient is created to convert all the films to absolute dose with in-house
build software called IMRTool. This software allows quantitative comparison of 2D
dose distributions using the gamma index calculation. The constraints used in our
department for gamma index calculations are 3% of the local dose and 3mm for
distance with an agreement score of 90%. In less than 2% of the cases the
agreement score is lower than 90%. In one case the speed of one leaf was incorrect.
For the other cases the agreement score was within the 90% if the QA procedure
was repeated.Over the years this procedure was optimized but still it takes about 12
minutes of machine time (with two dosimetrists) and about 80 minutes of processing
data to execute the whole procedure. Since June 2005 we can acquire dosimetric
electronic portal images. As the dosimetric information is readily available after
irradiation no processing time is needed other than a comparison between the
measurements with the EPID and a prediction of the EPIDs signal calculated in the
TPS. At this moment this measurement is scheduled in a short treatment machine
slot of six minutes (and one dosimetrist) and the evaluation process is performed
semi-automatically by the IMRTool software.
Keywords : IMRT, QA, patient
26 – ESTIMATING A CONFIDENCE LIMIT FOR THE INTEGRAL
EVALUATION OF IMRT TREATMENTS FOR HEAD & NECK TUMOURS.
B. Schaeken (1,2), B. De Ost(1), D. Van Gestel(1), D. Van den Weyngaert(1).
(1) Department of Radiotherapy, ZNA- Middelheim, Antwerp, Belgium
(2) Nuclear Technological Centre NuTeC, Xios Hogeschool Limburg, Hasselt.
MATERIAL AND METHODS – The alanine dosimeters consist of finely dispersed La-alanine crystals (75%w) suspended uniformly in a matrix of cellulose (17%w) and
additives to facilitate the process of direct tabletting using an eccentric press. The
dosimeters are read out with an X-band desktop EPR spectrometer (Bruker,
EMS104) with a cavity operating in the TM110 mode. The dosimeters have a
diameter of 4.9 mm to fit within the quartz holder of the resonant cavity and are 1 mm
to 5 mm long (mass density of 1302 kg/m3 ± 0.5% (k=1)). Dose is measured as the
peak to peak height of the central line in the absorption spectrum. The alanine
detectors show favorable dosimetric properties: linear dose response, no energy or
dose rate dependence, no fading and tissue equivalence. The combined uncertainty
is 2.2% (k=1) @ 6 Gy and rapidly decreases with increasing dose. IMRT treatments
are checked with the “copy to phantom” methodology.
RESULTS – 225 alanine measurements were performed for 26 patients which
received IMRT treatment for head & neck tumours. The calculated- to measured
dose was 1.016 ( s =4.0%). If we restrict data to patients treated with less than 50
beam segments, Dcal/Dmeas = 1.008 ( s =3.6%). To study the influence of daily
setup errors to the effectively delivered dose, the results of a 1 time 20 Gy set up was
compared to a 10 times 2 Gy set-up for the same treatment. For the latter, our
treatment team was installing the phantom each day on the treatment table, as they
would do in clinical routine, to administer 2Gy each session. In this way a total dose
of 20 Gy was accumulated in the detectors. The ratio of the calculated dose to the
measured dose was 0.991 (N=29; sd =5.1%) versus 1.011 ( sd =2.8%) for the 1 x 20
Gy and 10 x 2 Gy respectively. In the 1 x 20 Gy set up, 4 measuring points were
found outside the 5% tolerance level, for the 10 x 2 Gy experiment this was only the
case in 2 measuring points. Verification of the delivered dose in the treatment of
whole breast cancer shows that Dcal/Dmeas = 1.00 (N=121; sd =0.03). For the
treatment of brain tumours: Dcal/Dmeas = 1.01 (N=41; sd =0.015).
CONCLUSION – Alanine/ESR dosimetry has proven to be a reliable technique that
facilitated the introduction of complex treatment modalities at our department.
Numerous dose measurements allowed us to establish a confidence limit for the
whole treatment in function of the degree of modulation (complexity).
Keywords : confidence limit, IMRT, alanine dosimetry
27 – MONIQA, MONITOR QUALITY ASSURANCE: VALIDATION AND
INITIAL RESULTS OF A DYNAMIC PATTERN FOR DAILY QUALITY
CONTROL OF MEDICAL SCREEN DEVICES
Jurgen Jacobs, Tom Deprez, Frank Rogge, Hilde Bosmans
UZ Leuven, Gasthuisberg
LUCMFR
PURPOSE – We have developed a complete environment (MoniQA) for quality
control of all types of radiological screen devices, including specific viewing stations
for digital mammography. The European Protocol for quality control of digital
mammography systems (EUREF) prescribes tests for viewing stations, including
daily tests. Because these proposed tests have limitations (static patterns, learning
effect, difficult to use), we decided to develop a new dedicated pattern for daily
quality control. We report on the practical workout and on the first validation of this
pattern against the patterns of well-known protocols like the ones proposed by the
AAPMtg18 and the DIN.
METHODS AND MATERIALS – MoniQA was developed in JAVA. Besides a main
application to show patterns and to collect user data, it also consists out of a
framework to simplify the creation of new patterns. We implemented all patterns
needed by the AAPMtg18 protocol, by the DIN protocol and also a new dedicated
pattern which we call the MoniQA pattern. This pattern contains all elements
necessary to perform a full daily quality control: luminance, resolution, geometric and
general image quality checks. This is a dynamic pattern. Every time another pattern
is created according certain rules. This makes sure that there is no learning effect for
the observer. We compared the results of the different elements in the protocols and
the needed time. Four observers (both expert and novice) performed the three
already mentioned protocols on twelve different monitors. We included high end,
5MP monitors for mammography as well as screen devices for general radiology in
our test. The observations were made under clinical conditions.
RESULTS – We tested our software tool in a medical environment and it proved to
be stable. The comparison of the different results indicated that our pattern is able to
show at least the same possible malfunctions. In overall we saw that an evaluation of
the MoniQA pattern only took 50% of the time of the AAPMtg18 protocol and resulted
in a time gain of 15% compared with the DIN protocol.
CONCLUSION – Our preliminary results demonstrate that with our new pattern we
can easily and quickly monitor the quality of medical screens on a daily basis. The
results of our tests show that our approach gives at least the same results as the
well-known protocols. Combined with our software environment, we implemented a
powerful framework to perform the quality control of screen devices.
Keywords : monitor quality assurance
28 – DTI OF NORMAL APPEARING SPINAL CORD IN ELDERLY
Van Hecke W, Leemans A, Sijbers J, Parizel PM, Van Goethem JWM
University Hospital Antwerp
PURPOSE – Like the brain, the spinal cord contains many sensory and motor
pathways that can be visualized with diffusion tensor imaging (DTI). The DTI
technique measures the self-diffusion, or random thermal motion, of the endogenous
water in nerve tissue. Our objective was to evaluate quantitative DTI values of the
spinal cord in an older patient population with normal appearing spinal cord and
without spinal cord symptoms.
METHODS – Diffusion tensor measurements of the cervical spinal cord were
performed on 17 patients referred for degenerative spinal disease. All patients had a
normal appearing spinal cord on MRI and none had clinical spinal cord symptoms.
The mean patient age was 50.3 ± 10.8 years (min – max years). Acquisition
parameters were as follows: TR: 10.4 s; TE: 100 ms; diffusion gradient: 40 mT/m;
number of slices: 60; b: 700 s/mm; TA: 12” 18’; diffusion measurements in 60
directions. The cervical spinal cord was selected by manual delineation of an ROI on
all axial DTI slices. The data were analyzed using a propriety software tool
“ExploreDTI” developed at our institution. Fiber tracking was performed using
different values of fractional anisotropy (FA) for seed point selection and also to end
fiber tracking. The mean FA and mean diffusivity (MD) were calculated for the entire
cervical spinal cord. Statistical analysis was performed using a Mann-Whitney UTest.
RESULTS – A mean FA of 0.572 ± 0.024 was found with parameter settings of an FA
for seed point selection of 0.7 and an FA to stop tracking of 0.3. Under the same
conditions a mean MD of (1.08 ± 0.08) 10-3 mm2/s was found.
CONCLUSION – FA values of the cervical spinal cord in our patient population are
lower than other values published in the literature. We postulate that this might be (in
part) related to patient age. Studies in the brain showed a decrease of the FA in older
people [1]. In the future we will enlarge the control group and incorporate younger
people to investigate the exact relationship between the age and the quantitative DTI
parameters. In addition the influence of multipler sclerosis (MS) on the quantitative
DTI parameters of the cervical spinal cord will be studied.
[1] Salat DH, Tuch DS, Greve DN, van der Kouwe AJ, Hevelone ND, Zaleta AK,
Rosen BR, Fischl B, Corkin S, Rosas HD, Dale AM: Age-related alterations in white
matter microstructure measured by diffusion tensor imaging; Neurobiol Aging. 2005
Aug-Sep;26(8):1215-27. Epub 2004 Dec 19.
Keywords : diffusion tensor imaging, spinal cord, quantitative measures
29 - CHARACTERIZATION OF THE CR SYSTEMS.
Joris Nens
VUB
GOAL – To characterize the response of computed radiography (CR) systems for
different beam qualities.
MATERIALS AND METHODS – Two CR-systems of Kodak (CR400 and CR800) are
tested on 3 X-ray tables. On all tables, the response curves have been made. To see
the dependence of the response to the energy spectrum, the IP has been exposed
with the same dose for different kV and filter conditions. For both CR-systems the
differences between the cassettes has validated.
RESULTS AND DISCUSSION – For the response curves, differences were measure
of 1.6% for the slope and 0.4% for the offset, for the CR800 exposed with different
tubes but read on the same CR-reader. The response varies 12% when the kV is
changing in a range of 40 to 100 kV and about 5 to 6% when changing for no filter up
to 6 mm Al. The differences in response between the different cassettes are 0.84%
for the same type and 1.52% for all the cassettes together. These tests are the first
stage in the development of an automatic quality control.
Keywords : CR, quality control
30 – FORENSIC MEDICAL RADIATION PHYSICS IN BELGIUM.
P. Vander Henst (1,2,3), H. Bosmans (1,2), G. Marchal (1), T. Harding(3)
(1) Department of Radiology - UZ Gasthuisberg
(2) LUCMFR
(3) Institute for Forensic Medicine – University of Geneva
PURPOSE – Notwithstanding the fact that “forensic science” speaks to everyone’s
imagination, it still seems to be far away from the daily activities of the medical
radiation physicists. A theoretical analysis should give an answer to the question
whether medical radiation physics could be “highlighted” as a needed speciality
within the forensic science.
METHODS AND MATERIAL
Theoretical analysis
 in relation with daily routines
Although the ED97/43 entitle him as an expert in radiation physics and
technology applied for medical exposure, the medical radiation physicists do
not appear to consider the fact that their well defined responsibilities could
make them face liability charges.
 in relation with the first mandatory appointments by courts
The first mandatory appointments in Belgium indeed stated the in theorem.
for 5 cases by courts.
Peer Review Method
 The peer review method has been used to compare medical radiation physics
as a forensic science to other closely related forensic sciences and their
methods.
RESULTS – The theoretical outcome of the previous methods has been reassessed
against national and international publications, and was put into the legal, socioeconomic and ethical reality. It revealed a possible direct involvement at different
levels of the medical radiation physicist in forensic science:
- in a legal proceeding
- in an expertise
- the routinely given expert information
The results from the peer review revealed that indeed medical radiation physics was
recognised as an important part in multidisciplinary medico-legal forensic cases.
CONCLUSION – The result of the study got directly confirmed in daily situations
Keywords : forensic radiation science
31 – COMPARING DIFFERENT COLLIMATION TECHNIQUES (M3,120
MLC,CIRCULAR ARC) IN STATIC AND DYNAMIC ARC THERAPIE FOR
INTRACRANIAL STS
Joeri Alberty, G. Pittomvils, Y. Geussens, P. Herregodts, H. Verstraete, R.
Weytjens, A. Sprangers, E. Bossuyt, N. Mertens, R. Reymen , P. Huget.
AZ Sint-Augustinus GVA Wilrijk
INTRODUCTION – Since 1996 more then 400 patients are treated at our department
with Stereotactic surgery. Since 2003, after a Varian Clinac 2100 EX with 120 MLC
was installed and commissioned for circular arc and multileaf static arc. From that
moment on, stereotactic surgery is done with both methods.
PURPOSE – Comparing 5 different collimation techniques using Circular Cones in
static arc mode and the Brainlab M3 (3-5 mm) and Varian 120 MLC (5 mm)
multileaf collimator in static and dynamic arc mode.
METHODS AND MATERIALS – The patient data set is divided on pathology ,
localization , shape and size. Each patient is retrospectively replanned using circular
arc, conformal arc and dynamic arc treatment technique using the Varian 120 MLC
and the Brainlab micro-MLC using the BrainSCAN 5.31 planning software. The same
fixed, non coplanar beams are used for all plans (Isocenter , number of arcs , start
and stop position). The prescribed dose (80 % of the maximum dose) on the CTV
and the maximum tolerated doses on the organs at risk are identical as well. The
isodose distribution and dose-volume histograms (DVH) are computed and plans are
compared in terms of conformity, Dose received to the CTV , organs at risk and the
surrounding tissue.
RESULTS – It is feasible to plan with different conformal techniques and choose the
best option for individual treatment. Changing a plan from circular cone to conformal
arc is time consuming due to software limitations. This makes it necessary to
evaluate the size , shape and OAR before planning and choose the best option.
RESULTS INDICATE THE FOLLOWING OUTCOME – The minimum target dose
differs significantly where the M3 will be the best, the 120 MLC second, the circular
arc technique third due its circular shape. The total brain volume receiving 50 % of
the maximum dose is smallest for the circular cones, the largest for the 120 MLC.
The conformity index of the M3 treatments will be the best for the M3 dynamic
treatments and the 120 MLC treatments will have e better CI then the circular arc
treatments.
CONCLUSION – We are convinced that at the end of the study, we will be able to
give an answer on the question of the importance of leaf width and treatment mode
of MLC against the previous standard. A cutoff value when to use which technique
will be found. This should help any reader the determine himself the combination that
is the most suited for its centre.
Keywords : stereotactic intracranial treatment, collimation techniques, static/dynamic
arcs.
32 – INTENSITY MODULATED PHOTON (IMXT) AND PROTON (IMPT)
THERAPY FOR THE TREATMENT OF HEAD AND NECK TUMORS.
Steneker MNJ (1,2); Lomax AJ (1); Schneider U (3);
(1) Dept. of Radiation Medicine, Paul Scherrer Institute, CH
(2) University of Groningen, Groningen, NL
(3) Triemlispital, Zürich, CH
OBJECTIVE – A comparative treatment planning study has been preformed between
intensity modulated photon (IMXT) and intenstity modulated proton (IMPT) therapy to
investigate the ability of both modalities to spare critical organs in the head and neck
region while keeping target dose homogeneous. Additional advantages of variable
energy IMPT was also investigated. The treatment planning comparison was
extended by varying the number of fields in order to study its effect on the
performance of each modality, and risks of secondary cancer induction were also
calculated for all plans.
MATERIALS AND METHODS – Five planning CT's were selected for the study.
Four different constraints were set to the organs at risk, in order to measure the
resulting homogeneity of dose in the target volume. 5 and 9 field plans were made for
IMXT and 3, 5 and 9 field plans were made for IMPT and variable energy IMPT.
Dose homogeneity as a function of the constraints were visualized using a 'pseudo'
Pareto-optimal front approach. Risks of secondary cancer were estimated using the
organ equivalent dose model.
RESULTS – Critical organs were best spared using 3-field IMPT and, at least for
IMPT, little advantage was seen with increasing field numbers. In contrast, there was
a significant advantage in going from 5 to 9 fields for IMXT. Secondary cancer risk
was lowest for the IMPT variable energy plans, in which normal tissue received the
least integral dose. Interestingly, although integral dose remained the same,
increasing the number of IMPT fields increased the secondary cancer risk, due to the
increased volume of tissue irradiated to low dose.
CONCLUSION – IMPT has a better ability to spare critical organs than IMXT for the
same dose homogeneity. It also significantly reduced the predicted risk of secondary
tumour induction, and the use of small numbers of fields further increased this
advantage. Given that target homogeneity and normal tissue sparing were equally
good with the 3 field IMPT, there appears a clear rationale to deliver small numbers
of fields for IMPT.
Keywords : IMRT, protontherapy, secondary cancer
33 – THE MEDICAL RADIATION PHYSICIST TAKING PART IN
MEDICOLEGAL PROCEDURES: THE FIRST BELGIAN TENDENCIES
AFTER ONE YEAR EXPERIENCE.
P. Vander Henst (1,2,3), H. Bosmans (1,2), G. Marchal (1), T. Harding(3)
(1) Department of Radiology - UZ Gasthuisberg
(2) LUCMFR
(3) Institute for Forensic Medicine – University of Geneva
PURPOSE – The need for qualified radiation experts in medico-legal forensic
procedures is high; however the interest and the awareness of finding themselves
involved are still low. Being a forensic radiation physicist sounds attractive, but there
is a whole world of interesting topics beyond this imaginative idea. Lot’s of victims are
surviving an incident, and are in need for qualified persons to evaluate their
sustained human damage and to pinpoint the responsible.
Wanted: qualified expert! As the past year, 21 cases had been appointed to them by
the Belgian courts. They faced quite a lot of questions, as no standard rules existed.
The purpose of this study was to highlight a standard procedure.
METHOD & MATERIALS
Two tendencies could be differentiated.
The first trend is the so called ‘pure’ medico-legal proceedings.
The second one could be categorised into:
 claims submitted to the Council of State
 general radiation protection topics
 legal entities lodging appeal.
The framework for standard procedures will be set-up together with international
experts in other forensic disciplines. Called: multidisciplinary approach!
RESULTS – Adapting to the national context made the creation of the standard
procedure complicated. It describes the ways of how an expert could get involved
and the expectations towards them. Notwithstanding one general rule came directly
out of the study:
NEVER exceed your competence!
WATCH out for competence conflicts!
NEVER be a judge in your own cause!
CONCLUSION – The standard procedure reduced the threshold for the medical
radiation physicist to take up a role in proceedings.
Keywords : forensic radiation science
34 – PHASED ATTENUATION CORRECTION IN RESPIRATION
CORRELATED CT/PET
C.C.A. Nagel (1), G. Bosmans (1), A.L.A.J. Dekker (1), M.C. Öllers (1), D.K.M. De
Ruysscher (1), P. Lambin (1), A.W.H. Minken (1), N. Lang (2), K.P. Schäfers (2)
(1) Department of Radiation Oncology (MAASTRO), University Hospital Maastricht,
Maastricht, The Netherlands
(2) Department of Nuclear Medicine, University Hospital Muenster, Muenster,
Germany
PURPOSE – Motion of lung tumors with respiration causes difficulties in the imaging
with CT and PET. Since accurate knowledge of the position of the tumor and the
surrounding tissues is needed for radiotherapy planning it is important to improve
CT/PET image acquisition. The purpose of this study was to evaluate the potential to
improve image acquisition using phased attenuation correction in respiration
correlated CT/PET, where data of both modalities was binned retrospectively.
METHODS AND MATERIALS – Respiration correlated scans were made on a
Siemens Biograph Sensation 16 CT/PET scanner which was modified to make a low
pitch CT scan and list-mode PET scan possible. A lollipop phantom was used. The
sphere with a diameter of 3.1 cm was filled with approximately 20 MBq 18F-FDG.
Three longitudinal movement amplitudes were tested: 2.5, 3.9 and 4.8 cm. After
collection of the raw CT data, list-mode PET data, and the respiratory signal CT/PET
images were binned to ten phases with help of in house built software. Each PET
phase was corrected for attenuation with CT data of the corresponding phase. For
comparison, the attenuation correction was also performed with non-respiration
correlated (non-RC) CT data. The volume of the sphere and the amplitude of the
movement were calculated for every phase of both the CT and PET data (with
phased attenuation correction). Maximum and average activity concentrations were
compared between the phased and non-phased attenuation corrected PET.
RESULTS – With a standard, non-RC CT/PET scan, the volume was underestimated
by as much as 46% in CT and the PET volume was overestimated to 370%. The
volumes found with RC-CT/PET scanning had average deviations of 1.9% (± 4.8%)
and 1.5% (± 3.4%) from the actual volume, for the CT and PET volumes respectively.
Evaluation of the maximum activity concentration showed a clear displacement in the
images with non-RC attenuation correction, and activity values were on average 14%
(± 12%) lower than with phased attenuation correction. The standard deviation of the
maximum activity values found in the different phases was a factor 10 smaller when
phased attenuation correction was applied.
CONCLUSION – In this phantom study we have shown that a combination of
respiration correlated CT/PET scanning with application of phased attenuation
correction can improve the imaging of moving objects and can lead to improved
volume estimation and a more accurate localization and quantification of the activity.
Keywords : respiration correlation, attenuation correction, CT/PET
35 – IMAGE QUALITY IMPROVEMENTS FOR WHOLE BODY PET WITH
TIME-OF-FLIGHT
Stefaan Vandenberghe
Ghent University, ELIS/MEDISIP, Sint-Pietersnieuwstraat 41, B-9000 Ghent, Belgium
Recently there has been a renewed interest and development in Time-of-Flight (TOF)
PET based on fast scintillators with a sufficiently high stopping power and high light
output like LSO and LaBr3. The measured time difference in 3D TOF PET makes it
possible to improve the signal-to-noise ratio of reconstructed images. The
improvement in signal-to-noise ratio can be used to reduce imaging time or to
improve the quality of scans of eg. obese patients. It is well known that the reduction
in imaging time is proportional to the ratio of D/dx with the D the diameter of the
object and dx the Full Width at Half Maximum (FWHM) of the TOF resolution.
Expected reductions in imaging time for a 30 cm diameter object are 3.3 for 600 ps
timing resolution and about 6.6 for 300 ps timing resolution. Results will be
presented from a full ring LaBr3 based TOF PET scanner currently under
development at the University of Pennsylvania.
Reconstruction algorithms have to be modified to take the timing resolution into
account. Forward and back projection is still done along the same lines as in
conventional PET but all points along the line are now weighted with a different
probability. The maximum probability is given to the point determined by the
measured TOF difference and the width of the probability distribution is determined
by the expected accuracy of the time difference measurement. Listmode
reconstruction seems to be the most obvious choice for TOF-PET, but will result in
relatively long reconstruction times. Therefore we have developed a rebinning and
mashing algorithm to transform the data to a limited number of 2D datasets. It is
shown that images can be reconstructed from less angles when timing information
improves. Using these rebinning and mashing algorithms allows the use of fast 2D
reconstruction algorithms.
In iterative reconstruction convergence is non linear and depends on the local
environment and the object size. To obtain uniform convergence in PET studies one
method is to iterate very long and postfilter the images. It is shown with some nice
examples that this non-linear behavior improves when TOF information is used. In
general convergence improves with better timing resolution and it becomes less
dependant on the local environment and the object size. Therefore it can be
expected that TOF PET will also help in quantification and diagnosis of iterative
reconstructions.
36 – THE “TIBIT”: PRODUCT DESIGN OF A PATIENT POSITIONING
SYSTEM FOR TOTAL BODY IRRADIATION
S. Van Boeckel (1), J. Vanregemorter (2), Ch. Baelus (1)
(1) Hoger Instituut voor Integrale Product Ontwikkeling, Hogeschool Antwerpen.
(2) ZNA Middelheim hospital, Department of Radiation Oncology
PURPOSE – This abstract summarises a thesis made in 2005 at the Department of
Consumer Goods. The goal was to design a positioning aid for fractionated total body
irradiation including transmission blocks for lung shielding and the possibility to make
megavolt images using (CR) cassettes.
PRODUCT IDEA – The main drivers for the design were: time reduction, reduction of
effort and patient comfort. The main developed items are: A] reproducible, reliable
and comfortable body support with fixation of the thoracic region, B] reproducible and
reliable shielding of the lungs. Secondary items: system storage and mobility, access
to the patient, communication with the patient and connectivity. Extra items:
dimensions of the irradiated area, patient position, PMMA radiation diffuser, no radioopaque material between source, patient and cassette, recognition of patient
dependant parameter settings (eg fixation, shielding), patient friendly and easy to
clean.
SYSTEM DESIGN – Various existing methodologies were studied and tested. A cost
analysis was made as well as a market study.
PRODUCT CONCEPT – The unit consists of a rotating stand on a fixed base plate.
The front of the unit supports the PMMA radiation diffuser and an adjustable block
system. The patient rests on a reclining surface in carbon to which panels holding the
thorax fixation, bottom and head rests are connected. The panels fit in a unique way
to the surface. The back of the unit can hold a standard cassette at a selectable
height. A number of switches interlocks to the linac to prevent erroneous irradiation.
The patient has audio-visual communication with the control room and the progress
of the treatment can be followed by the patient on a display in the unit. All cables are
integrated in the frame of the unit including connections for in-vivo diodes.
The base plate locks into position and is not moved during treatment. For APPA
treatment, the stand is rotated maintaining the patient position in the stand, the
diffuser, block holder and cassette holder remain steady. The Tibbit unit can be
stowed away when not in use and fits through a standard size door.
CONCLUSION – The design tackles most problems that are encountered with Total
Body Irradiation. The use of the Tibbit system would add to the quality of the patient’s
treatment and reduce handling time by the nursing staff. The poster shows the
design with detailed views of the various parts.
37 – SUPER ABSTRACT VAN MARC COGHE (WIP)
Marc en zonen
UZ Gent
xyz
Keywords : xyz
38 – REPEATED (RC)CT IMAGING DURING A COURSE OF
RADIOTHERAPY FOR LOCALLY ADVANCED NSCLC PATIENTS.
G. Bosmans, A. van Baardwijk, A. Dekker, M. Öllers, L. Boersma, P. Lambin, A.
Minken, D. De Ruysscher
Department of Radiation Oncology (MAASTRO), GROW, University Hospital
Maastricht, Maastricht, The Netherlands
PURPOSE – Radiation treatment planning is mostly based on one single CT scan. A
respiration correlated (RC) CT scan can improve the imaging of lung cancer patients.
The aim of this study was to investigate the changes in tumor volume and tumor
mobility during a course of radiotherapy for locally advanced non small cell lung
cancer patients and whether repeated imaging is necessary to adapt the treatment
plan.
METHODS AND MATERIALS – Images were acquired on a modified Siemens
Biograph 16 CT/PET scanner. Twenty-five patients got a conventional CT prior to
treatment and one and two weeks after the start of the treatment. For thirteen
patients also an RCCT was acquired at the three timepoints. Tumor volumes were
drawn for each patient by the same physician, to avoid interobserver variability.
Tumor movements were measured in all three orthogonal direction using in-house
developed Matlab software. Patients were treated twice a day with 1.8 Gy to ensure
a short overall treatment time.
RESULTS – On average the 3D vector of the tumor movement prior to treatment was
7.1 mm ± 4.5 mm wich changed to 7.0 mm ± 5.9 mm and 7.7 mm ± 5.5 mm one and
two weeks after the start of the treatment respectively. Breathing frequencies were
similar at all three timepoints because the RCCT acquisition was done after the
CT/PET scan, so the patients lied down during approximately 2 hours and were
completely relaxed. Tumor volumes on CT increased during radiotherapy treatment.
The CT volume, relative to the volume prior to treatment increased to about 125%
and 107%, one and two weeks after the start of the treatment. This is probably due to
the fact that inflammation and atelectases occurs, since after one week already
approximately a radiation dose of 20 Gy is given.
CONCLUSION – Tumor mobility did not change during a course of radiotherapy in
this patient population, so repeated RCCT is not necessary during treatment. But the
tumor volume on CT did change during the treatment, more (biological and
molecular) information is needed to investigate whether this is real tumor progression
or radiation reaction.
Keywords : respiration correlation, repeated imaging, lung cancer
39 – BREATHING CURVE PRODUCED BY MAGNETIC SENSORS
Michel Destiné (1), Vincent Remouchamps (1), Pierre Ansay (2), Bernard
Beckers (2), Dominique P. Huyskens (3), Emile Salamon (1)
(1) Radiotherapy Department, Namur, Belgium
(2) Nomics, Liège, Belgium
(3) 7Sigma, Leuven, Belgium
PURPOSE/OBJECTIVE – To describe the development and the testing of a
breathing curve produced by a magnetic distance meter. The aim is to develop a
simple, patient friendly, easy to use and not expensive system that will be used to
monitor voluntary breath hold.
MATERIALS/METHODS – A magnetic distance meter (Nomics, Belgium) is used to
obtain the distance between two resonant circuits with an accuracy below 0.2 mm.
This distance meter uses the property of a resonant circuit to excite another circuit,
matched on the same frequency through their mutual inductance. The use of
resonant circuits instead of stand-alone inductances greatly improves both the
performance of the excitation circuit and the sensitivity of the sensor. Resonant
circuits are matched to a frequency of the order of 8 kHz and are protected in a 8 x 8
x 27 mm plastic cap. The emission inductance is inserted in a series resonant circuit
stimulated at a cadence of 20 Hz. The signal thus consists of a succession of
separated, damped sinus waves. The reception circuit restores the signal of the sinus
pulses observed on the parallel resonant sensing circuit.
RESULTS – The first resonant circuit is taped on the lateral side of the thorax and
the second resonant circuit is taped on the immobilization device or on the treatment
table. The baseline distance is typically around 10 cm. The sensors alone were
submitted to direct and scattered 6 and 18 MV photons to quantify signal changes by
the electro-magnetic field induced by the Varian 2100. During free breathing, the
distance variation is in the range of 3 to 4 mm, producing a meaningful breathing
curve. Deep inspiration breath hold produces a larger distance difference, 1 to 1.5
cm. Results of the preliminary 1-D analysis correlating the breathing curve and the
set up errors on movie portal images will be updated.
CONCLUSIONS – A breathing curve using thoracic magnetic sensors is successfully
produced with a user-friendly system. In terms of quality control, the system has the
potential to monitor the irradiation window during breath hold treatment. The 3-D
reproducibility will be tested in a future protocol.
Keywords : magnetic sensors; voluntary breath hold; breathing curve
40 – RETROSPECTIVE DOSIMETRIC ANALYSIS OF PDR TREATMENT
PLANNING IN CERVIX BRACHYTHERAPY BASED ON 3D MR IMAGING
Marisol De Brabandere, An Nulens, Amr Mousa, Erik Van Limbergen
University Hospital Gasthuisberg
In the Radiation Therapy Department of Leuven, PDR treatment planning of cervix
cancer brachytherapy is traditionally based on X-ray films. The dose is prescribed to
point A; the total prescription dose is as high as possible, limited by the tolerance
doses and dose rates to the organs at risk. These tolerance doses are assessed by
anatomical points (bladder and rectal points) that are defined and digitized on the
films as recommended in ICRU38.
Since 2002, 3D MR images are acquired with the applicator in place in addition to the
X-ray films. Before basing our planning fully on this new 3D information, we included
a transition period during which we continued to perform treatment planning
according to our film approach. The 3D anatomical information was used solely for
modest modifications in source and dwell time setup in case of clear underdosage of
the target region or overdosage of a critical organ. The aim of this study is to
evaluate retrospectively the dose delivered to the target and the critical organs as
projected on MRI.
17 consecutive patients with cervical cancer (stage I to IV) treated with PDR during
the transition period are selected. Besides bladder, rectum and sigmoid, three target
volumes, i.e. GTV, CTV Intermediate Risk (CTV-IR) and CTV High Risk (CTV-HR),
are delineated on Plato (Nucletron) following the GYN GEG-ESTRO WG
recommendations [1].
Correlations are investigated between the traditional dose prescription at point A and
target coverage. The latter is quantified with the dose volume parameters D90, D100
and V100 as proposed by the GYN GEC ESTRO WG, as well as V(60GyEQD2) and
V(85GyEQD2) [2]. V(60GyEQD2) plays a role in the evaluation of CTV-IR as an
equivalent for the more general 60 Gy reference volume defined for LDR and
V(85GyEQD2) reports on a dose which represents more closely the prescription
dose to CTV-HR. Total doses of EBT and BT are considered in terms of biologically
equivalent dose (EQD2 indicates the isoeffective equivalence to 2Gy/fraction). For
evaluation of the OAR, the dose volume parameters D0.1cc, D1cc, D2cc, D5cc are
compared to the bladder and rectal ICRU reference points.
This analysis helps us to interpret 3D image based treatment planning and to further
optimize the dose distribution in terms of target coverage and dose reduction to the
organs at risk.
[1] Haie-Meder et al. R&O, 2005;74:235-245
[2] Pötter et al. Submitted to R&O, 2005
Keywords : cervix, brachytherapy, 3D planning
41 – COPY TO PHANTOM: A TOOL AND A TOY?
B. De Ost, B. Schaeken, D. Van Gestel, A. Coelmont, D. Van den Weyngaert
ZNA Middelheim, Antwerp, Belgium.
With the ‘copy to phantom’ (CTP) procedure, the beam set-up of an individual patient
is copied to a phantom. The dose must be recalculated for the new data set; dose
measurements can be performed in the phantom and compared with calculations.
We use this procedure in routine for all invers IMRT patients (Schaeken et al.,
submitted for publication in R&O). We found this to be a very powerful tool and
started to use it for other purposes as well:
1. Comparison of the alanine measured dose and the Philips (Pinnacle3) calculated
dose for a conformal 7-field treatment in a sinus carcinoma (Claus et al., 2001).
Despite the large air gaps and the complicated beam set-up, the measurements
proved that the calculations are reliable: for a total of 14 alanine measurements
the DD (dose difference, ratio calculated and measured dose) is 1.00,
1SD=2.2%.
2. Confirmation of the dose in the match plane of adjacent fields in head and neck
patients. A mono isocentric treatment of 2 lateral and 1 supraclavicular beam was
copied to the Alderson phantom, 54 TLD’s were positioned in the matching zone,
the mean DD was 1.00 (1SD=3.9%). For a ‘classic’ 3 point SSD set-up, with the
table rotated to align the supraclavicular field with 2 opposing lateral fields the
mean DD in the matching zone is 0.98 (N=30, 1SD=3.8%).
3. Copying an IMRT beam set-up to a new CT scan: during IMRT treatment the
anatomy of a head and neck patient changed so much that the radiation
oncologist decided for a new CT. To avoid starting the planning procedure all
over again, we copied the regions of interest to the new CT data set, saved this
as a phantom and copied the original beam set-up to this phantom. Finally we
evaluated the dose for the new clinical situation.
4. A forward planning is performed for the irradiation of the breast. Segments are
manually created covering the hot spots. In case adjacent lymph nodes must be
irradiated, the predicted dose scatter contribution of these fields to the mamma
must be correct if we want to compensate for this with a segment. A patient setup was copied to a rectangular phantom and doses were measured with TLD on
different distances and depths. The planning system overestimates the dose,
differences become larger with distance from the Co-60 field border, mean ratio
predicted dose, measured dose was 1.59 (1SD=79%). For 6MV beams the
scatter contribution is calculated correctly: mean ratio predicted and measured
dose was 1.00 (1SD=16%).
Keywords : quality assurance, copy to phantom.
42 – IVD IN TBI: PATIENTDOSE, LUNGDOSE AND REPRODUCIBILITY OF
THE TREATMENT.
B. De Ost, A. Coelmont, P. Meijnders, J. Vanregemorter, D. Van den Weyngaert
ZNA Middelheim, Antwerp, Belgium.
Total body irradiation (TBI) for ZNA (Ziekenhuis Netwerk Antwerpen) was clinically
started in October 2004. Until today 14 patients are treated with a schedule of 6
times 2 Gy in 72 hours using 18MV photons. Transmission blocks are used to reduce
the lung dose to 9 Gy. Quality control of the treatment is done with in vivo dosimetry
(IVD). A total of 836 measurements were performed in 84 treatments set-ups.
Two EDP20 diodes (Scanditronix) are calibrated for the Source Skin Distance and
field size used in TBI. The diodes are positioned on the central axis (cax) of the
treatment field and under the lung block. In this way the entrance dose and the lung
dose is controlled. The measurements are preformed for the anterior and posterior
field separately. After calibration of the TLD’s (Thermo 1/8” sq. chips; Harshaw 5500)
in treatment conditions the detectors are positioned on the same spots as the diodes;
extra TLD’s are placed on the head, sternum and leg. The TLD’s are packed in a
plastic bag. Home made water equivalent build-up caps of 10 mm thickness are used
for a selected number of patients. Most stable results are found with diodes: the
mean entrance dose measured is 1.57 Gy (1SD=3.5%, N=98); mean entrance lung
dose is 1.04 Gy (1SD=4.1%, N=98). The standard deviation for TLD measurements
with and without build-up is tripled in comparison with diodes. Using no build-up
material for the TLD measurements the dose on the lungs is overestimated because
of the scatter contribution of the lung blocks. The scatter effect of the lung blocks to
the dose in the build up region is analyzed on this moment to declare the differences
in value. The mean ratio of the lung dose and the cax dose with diodes is 0.66
(1SD=6.8%, N=133), for TLD without build-up 0.83 (1SD=6.4%, N=83) and TLD with
1cm build-up 0.74 (1SD=6.5%, N=25). Measurements in a rectangular phantom and
in the Alderson phantom result in a 75% lung dose in comparison with the dose on
the central axis. Reproducibility of the treatment can be traced by interfractional SD
of the measurements for 1 patient in one particular measuring spot. The mean SD for
these measurements is 6%; in 16 cases the SD is found outside 10% on a total of
129 measuring series. In vivo dosimetry in total body irradiation demands a special
calibration set-up. Reproducibility of the treatment is proved, but more details on the
scatter contribution of the lung block are needed to analyze the entrance dose
measurements correctly.
Keywords : total body irradiation, in vivo dosimetry, entrance dose measurements.
43 – A QUALITY ASSURANCE SOFTWARE NETWORK FOR RADIOLOGY
T Deprez, J Jacobs, G Marchal, H Bosmans
UZ Leuven - Radiology – LUCMFR
PURPOSE – Automated data collection and customizable data analysis for any
physical and/or technical quality controls for decentralised radiological centres.
METHOD AND MATERIALS – Two applications QASPR (Quality ASsurance for
Physics in Radiology) and QAMPR (Quality Assurance Management for Physics in
Radiology) were developed for the need of a global automated QA management.
They have been developed in Delphi and Java, respectively. Both applications are
set-up in a modular way, to address customization and extension easily. QASPR,
used at satellite centres, is the first line providing quality control feedback. It deals
with the collection of data from different sources for all performed tests, provides an
analysis and communicates with QAMPR in a locally preferred exchange protocol.
QAMPR manages the data from all centres and provides more extended means for
analyses by our physicists. In addition it handles reporting and functions as the
equipment information hub.
RESULTS – We have tested the software tools and in particular their modular
structure for several applications in practice: the use of different phantoms, different
quality controls, different densitometers, different exchange protocols, and the
application of different formulas to calculate the parameters to be supervised in
different countries.
CONCLUSION – The software showed to be very stable and easily adjustable to
particular applications, such as the different QA mammography phantoms used at
our centres, the parameters of the European guidelines for mammography screening
versus German DIN norms, data collection from different equipment and other
sources like our own software tools concerning QA of viewing stations and digital
mammography.
Keywords : QA, software, automation
44 – AN AUTOMATIC PLATFORM TO DETECT ANOMALIES IN
TREATMENT PLANNING: OLEANDER, THE GUARDIAN
Séverine Cucchiaro (1), Arnaud Ligot (1), Michel Destiné (1) , Dominique P.
Huyskens (2)
(1) Department of Radiotherapy, Namur, Belgium
(2) 7Sigma, Leuven, Belgium
PURPOSE – To develop a platform that detects inconsistencies in treatment plans
and sends a warning to the physicist or dosimetrist. These inconsistencies may either
be dosimetric errors or erroneous parameter values leading to problems in the further
treatment process.
MATERIAL/METHODS – The software program Oleander, written in C++, accesses
directly the database of Vision Generation 7 (Varian Medical System) of the
Radiotherapy Department of Namur. The program consists of a cohort of tests meant
to detect anomalies in the treatment planning.
The following categories of tests are available:



tests on treatment prescription: i.e. total dose, dose per fraction, energy
tests on treatment fields: i.e. monitor units, wedge, gantry and collimator
angles, tolerance table, etc.
tests on plans: sum of field weights, plan approval, etc.
The number and type of tests will increase as the platform evolves.
Oleander consists of two parts:


a configuration module: this is the initialization part of Oleander. The user
determines which tests will be activated for a given treatment site. For each
site, the user should enter a list of patients that will form the reference
population for that treatment site. If the nature of the test requires so,
statistics are established with the reference population.
the platform: new plans are sent to the platform where the tests are executed;
a report on the results of the tests is generated.
RESULTS/DISCUSSION – The first prototype of Oleander was developed. It has
been tested for breast treatments. About 30 tests have been developed. Although
further improvements are needed in terms of interface, Oleander has already proven
useful in detecting inconsistencies in treatment plans. Preliminary results will be
presented.
CONCLUSION – Oleander, the guardian, will not only help to detect errors in
treatment proposals, but it will also improve the efficiency of the process by detecting
inconsistencies up front.
Keywords : quality control; automation; radiotherapy
45 – QUALITY CONTROL OF PET AND PET/CT
Immesoete P, begeleider: Bacher K., promotor: Thierens H.
AV Controlatom
INTRODUCTION – According to the Belgian law, the criteria in the publication
Radiation Protection 91 for quality control of a gamma camera and isotope
calibrators are to be used. However, there are no recommendations given neither for
PET nor for PET/CT. As quality control of the equipment is one of the main tasks of a
medical physicist, the aim of this work was to look which tests were relevant for
image quality, which tests could easily be performed in routine and to establish a
camera independant tool to check the quality of the PET/CT-scanner. We will focus
on PET as the criteria for quality control of CT are well described in Radiation
Protection 91 and need to be used.
METHODS AND MATERIALS – The tests proposed are based on the NEMA NU 22001 publication where several tests for quality control for PET are mentioned. For
these tests specific phantoms are needed, a huge amount of activity is required and
the tests + analysis take about 24h. Therefore a more user-friendly way to check the
quality of the camera under circumstances used in daily clinical routine is proposed.
The Jasczackphantom was used for spatial resolution and uniformity (as for SPECTsystems) as well as for sensitivity and scatter fraction. The difference with the NEMAtests are that NEMA looks after the best possible results where we performed the
tests under clinical circumstances by using the routine reconstruction algorithm,
scatter correction method and randoms correction. As scatter material we used water
mixed with F-18. The spatial alignment phantom with 6 Na-22 point sources is used
to verify the alignment between PET and CT.
RESULTS – The tests and criteria that will be presented are only for PET except one
test that verifies the alignment between PET and CT:








alignment between PET and CT: max. 5 mm misalignment (visual control)
peak settings
transmission sinogram: no non-uniformities
emission sinogram: no large gaps, no fluid lines
spatial resolution: min. diameter of 7,9 mm need to be clearly visible
sensitivity: use baseline-value
scatter fraction: use baseline-value
integral uniformity: <20%
CONCLUSION: To establish a program for quality control of PET and PET/CT is
necesarry. Progress has been made but this work needs to be continued. The
proposed tests need to be performed on different types of scanners and need to be
compared to make a general Belgian proposal.
Keywords : quality control, PET/CT
46 – MONIQA, MONITOR QUALITY ASSURANCE: VALIDATION AND
INITIAL RESULTS OF A DYNAMIC PATTERN FOR DAILY QUALITY
CONTROL OF MEDICAL SCREEN DEVICES
Jurgen Jacobs, Tom Deprez, Frank Rogge, Hilde Bosmans
UZ Leuven, Gasthuisberg
LUCMFR
PURPOSE – We have developed a complete environment (MoniQA) for quality
control of all types of radiological screen devices, including specific viewing stations
for digital mammography. The European Protocol for quality control of digital
mammography systems (EUREF) prescribes tests for viewing stations, including
daily tests. Because these proposed tests have limitations (static patterns, learning
effect, difficult to use), we decided to develop a new dedicated pattern for daily
quality control. We report on the practical workout and on the first validation of this
pattern against the patterns of well-known protocols like the ones proposed by the
AAPMtg18 and the DIN.
METHODS AND MATERIALS – MoniQA was developed in JAVA. Besides a main
application to show patterns and to collect user data, it also consists out of a
framework to simplify the creation of new patterns. We implemented all patterns
needed by the AAPMtg18 protocol, by the DIN protocol and also a new dedicated
pattern which we call the MoniQA pattern. This pattern contains all elements
necessary to perform a full daily quality control: luminance, resolution, geometric and
general image quality checks. This is a dynamic pattern. Every time another pattern
is created according certain rules. This makes sure that there is no learning effect for
the observer. We compared the results of the different elements in the protocols and
the needed time. Four observers (both expert and novice) performed the three
already mentioned protocols on twelve different monitors. We included high end,
5MP monitors for mammography as well as screen devices for general radiology in
our test. The observations were made under clinical conditions.
RESULTS – We tested our software tool in a medical environment and it proved to
be stable. The comparison of the different results indicated that our pattern is able to
show at least the same possible malfunctions. In overall we saw that an evaluation of
the MoniQA pattern only took 50% of the time of the AAPMtg18 protocol and resulted
in a time gain of 15% compared with the DIN protocol.
CONCLUSION – Our preliminary results demonstrate that with our new pattern we
can easily and quickly monitor the quality of medical screens on a daily basis. The
results of our tests show that our approach gives at least the same results as the
well-known protocols. Combined with our software environment, we implemented a
powerful framework to perform the quality control of screen devices.
Keywords : monitor quality assurance
47 – IMAGE QUALITY PERFORMANCE OF LCD DEVICES: INFLUENCE
OF DISPLAY RESOLUTION, MAGNIFICATION AND WINDOW SETTINGS
ON CONTRAST-DETAIL DETECTION
Klaus Bacher (1), An De Hauwere (1), Peter Smeets (2), Philippe Duyck (2),
Koenraad Verstraete (2), Hubert Thierens (1)
(1) Ghent University, Department of Medical Physics and Radiation Protection
(2) Ghent University Hospital, Department of Radiology
The aim of this study was to investigate the combined effects of liquid crystal display
(LCD) resolution, image magnification and window/level adjustment on the low
contrast performance in soft-copy image interpretation in digital radiography and
digital mammography. In addition, the effect of a new LCD noise reduction
mechanism on the low-contrast detectability was studied.
Digital radiographs and mammograms of two contrast-detail phantoms (CDRAD 2.0
and CDMAM 3.4) were scored by a group of fifteen observers on five LCD devices
with varying resolutions (1-, 2- , 3- and 5-megapixel) and one dedicated 5-megapixel
cathode ray tube monitor. Two 5-megapixel LCDs were included. The first one was a
standard 5-megapixel LCD, the second had a new (Per Pixel Uniformity) noise
reduction mechanism. The contrast-detail images were analyzed in four different
interpretation sessions. In the first session, images were visualized using a standard
representation. Secondly, the observers were forced to use the interactive
window/level adjustment. In the next session, the images were presented at full
resolution but window/level adjustment was not allowed. Finally, observers scored
the images at full resolution together with the use of window/level adjustment. For
each image analysis, the interpretation times were registered.
A multivariate analysis of variance revealed a significant influence of LCD resolution
(p=0.01), image magnification (p=0.002) and window/level adjustment (p=0.001) on
the low-contrast image quality performance. The interactive adjustment of brightness
and contrast of digital images did not affect the reading time (p=0.10), whereas
magnification to full resolution resulted in a significantly slower softcopy interpretation
(p=0.008).
For digital radiography applications, a 3-megapixel LCD is comparable with a 5megapixel CRT monitor in terms of low-contrast performance as well as in reading
time. The use of a 2-megapixel LCD is only warranted when radiographs are
analysed in full resolution and when using the interactive window/level adjustment.
In the digital mammography setting, a 5-megapixel monitor should be the first choice.
In addition, the new PPU noise reduction system in the 5-megapixel LCD devices
provides significantly better results for mammography reading as compared to a
standard 5-magapixel LCD or CRT. If a 3-megapixel LCD is used in mammography
setting, a very time-consuming magnification of the digital mammograms would be
necessary.
Keywords : soft-copy display, digital radiography, digital mammography
48 – IMAGE QUALITY AND RADIATION DOSE IN DIGITAL CHEST
IMAGING: COMPARISON OF AN AMORPHOUS SILICON AND AN
AMORPHOUS SELENIUM FLAT-PANEL SYSTEM
Klaus Bacher (1), Ludo Vereecken (2), An De Hauwere (1), Peter Smeets (3),
Philippe Duyck (3), Robert De Man (2), Koenraad Verstraete (3), Hubert
Thierens (1)
(1) Ghent University, Dept of Medical Physics and Radiation Protection
(2) Heilig Hart Hospital Roeselare
(3) Ghent University Hospital
The aim of this study was to compare the image quality and the radiation dose in
chest imaging using an amorphous silicon and an amorphous selenium flat-panel
detector system. In addition, the low contrast performance of both systems with
standard and low radiation dose was compared.
In two groups of 100 patients each, digital chest radiographs were acquired either
with the amorphous silicon or the amorphous selenium flat-panel system. The
effective dose of the examination was measured using thermoluminescent
dosimeters placed in an anthropomorphic Rando phantom. The image quality of the
digital chest radiographs was assessed by five experienced radiologists using the
European Guidelines on Quality Criteria for Diagnostic Radiographic Images. In
addition, a contrast-detail phantom study was set up to assess the low contrast
performance of both systems at different radiation dose levels. Differences between
two groups were tested for significance using the two-tailed Mann-Whitney test.
The amorphous silicon flat-panel system allows an important and significant
reduction in effective dose in comparison with the amorphous selenium flat-panel
system (p<0.0001) for both the PA and lateral views. In addition, clinical image
quality analysis showed that the dose reduction was not detrimental to image quality.
Compared to the amorphous selenium flat-panel detector system, a significantly
better low-contrast phantom performance of the amorphous silicon detector system
was shown for phantom entrance dose values up to 135 µGy.
Chest radiographs using the amorphous silicon flat-panel system can be acquired
with a significantly lower patient dose compared to those made with the amorphous
selenium system, thereby producing an image quality that is equal to or even
superior to that of the amorphous selenium flat-panel detector system.
Keywords : digital radiography, flat-panel detectors, contrast-detail
49 – COMPARISON OF OPTIMISATION METHODS FOR INVERSE IMRT
PLANNING
Z.Beelen, E. Bressers, K.Bamps, P.Bulens
Virga Jesseziekenhuis / Limburgs Oncologisch Centrum (LOC)
In radiotherapy there are different techniques for applying a dose to the patient. One
of the advanced techniques used nowadays is Intensity Modulated Radiotherapy
(IMRT). During IMRT, a number of beams irradiate a target volume. Each beam is
divided in a series of subsequent segments defined by a multileaf collimator (MLC).
Before planning starts a number of objectives are assigned to the different volumes
of interest (VOI). For the organs at risk (OAR) these objectives are defined in terms
of maximum dose or as a dose volume constraint. For the target volumes a
prescribed dose is defined together with a maximum and/or a minimum dose.
To create an optimal IMRT plan it is necessary to make use of an inverse treatment
planning system (TPS). This system optimises the dose distribution until it fulfils the
most of the objectives and constraints.
To optimise the dose distribution the objectives and constraints are all incorporated in
an objective function. This function will have to be minimised (or maximised in some
cases) to find an optimal plan. At the moment there are several techniques
implemented in commercially available systems to find this optimum.
In this study a number of optimisation methods were compared: the gradient descend
method, the DMPO method in which the beams are immediately segmented and the
MLEM optimisation used in IMRS. The raw data of this comparison was examined
using ANOVA and MANOVA statistics. The results show significant differences
between the investigated methods at a 95% confidence level. The differences occur
as well in the number of monitor units (MU) needed to deliver a specific dose as in
the quality of the plans. The quality of the plans was expressed using a parameter
that incorporates the objectives and constraints for the Volumes of interest (VOI).
Keywords : inverse dose planning, IMRT, dose optimalisation
50 – ACCEPTANCE TESTING OF A TREATMENT PLANNIG SYSTEM
Belge D., Infantino S.
CHU Tivoli - Radiotherapy Dpt
PURPOSE - To present our acceptance tests with Eclipse TPS. Tangentiel fields :
work in progress !!
MATERIAL AND METHOD – Eclipse TPS, PBC algorithm V. 7.3.10 from Varian
Medical Systems. Measurements : Blue Phantom, ionization chambers IC13, Dose1
electrometer from Wellhöfer; polystyrene phantom; Kodak EDR2 films; Vidar VXR-16
DosimetryPro scanner, NE 2571 0.6 cc carbon chamber. Analysis softwares :
DoseLab 4, OmniPro-Accept 6.4. Beam : cobalt, open and wedged fields (30° and
45°). SSD ref. cond. = Varian recommendation (calculation algorithm optimized for
SSD basic measuments). Protocols : Estro booklet n°7 and NCS draft publication
"Quality assurance of 3D TPS".
RESULTS - PDDs OK for open and wedged beams (diff. < 0.1 %, tolerance 2 %).
Beams profiles :




Fields dimensions : OK for all open and wedged beams (diff. < 0.2 mm, tol. 2
mm).
High dose - high gradient areas : OK for open square fields from 4 to 20 cm²
and up until 10 cm depth. Larger sizes and depth : penumbras out the 2 mm
shift tolerance (20 % iso. shift : up to 3.1 mm 20 %; 80 % iso. shift : 3.6 to 9.5
mm for fields > 25 cm²). Wedges : 20 % iso. shifts increases with fields size
and depth (3 to 11 mm, thin edge only).
High dose - low gradients areas : OK for open fields, sizes > 6 cm² and 0.5 to
10 cm depth. Out of the 2 % tolerance for other cases (2.1 to 8.5 %) leading
to a shrinking of 90 % and 95 % isodoses. Wedges OK up to 5 cm depth, out
of tol. for larger depths (2.5 to 7.8%).
Low dose - low gradients areas : out of tolerance in all cases due to collimator
screws.
Special cases : tangential fields. Eclipse calculated doses in the "thin area" seem
very low, we are studying these cases with films. Inhomogeheities still have to be
studied. Treatment times : OK.
CONCLUSIONS – We are confident in Eclipse for calculations up to 10-15 cm depth.
For larger depths (unusual for a cobalt unit) we know we have to be careful due to 90
% and 95 % isodoses shrinking. Doses underestimation outside beam edges is
problematic in junctions (collimator screws). Still much work in progress for tangentiel
fields and inhomogeneities!!! Comparision of calculations with SSD and SAD basic
measurements is foreseen.
Keywords : TPS, dosimetry, acceptance
51 – THE USE OF ELECTRONIC COMPENSATORS TO OPTIMIZE DOSE
DISTRIBUTIONS IN BREAST TREATMENTS:
A FEASIBILITY STUDY AND FIRST TREATMENTS
Sabine Bernard (1), Ann Van Esch (2), Roxanne Delforge (1), Catherine Weber
(1), Emile Salamon (1)
(1) Department of Radiotherapy, Namur, Belgium
(2) 7Sigma, Leuven, Belgium
PURPOSE – To replace the use of conventional wedges in breast treatments with
electronic compensators generated by dynamic multileaf collimation in an attempt to
improve dose homogeneity
BACKGROUND – Breast patients in the Radiotherapy Department of Namur are
routinely planned –by using a 3D CT-scan- with conventional (hard and dynamic)
wedges in the medio-lateral direction combined with –in some cases- in the craniocaudal direction. The development of IMRT has opened the possibility to “redesign”
the concept of missing tissue compensators to further improve the dose
inhomogeneity. Varian has recently implemented such electronic compensators
(“irregular surface compensators”) into the Eclipse TPS. Standard IMRT requires
contoured volumes for the planner to run the optimization on. Electronic compensator
modules, on the contrary, require no other contouring than the body outline and
automatically generate the surface on which the dose volume optimizer aims to
deliver a homogenous dose. The optimal fluence produced as such is converted into
dynamic leaf motion files for delivery. Hence, the former mechanical compensators
are mimicked by the dynamic multileaf collimator.
MATERIAL AND METHODS – For a cohort of breast patients (N> 40), electronic
compensator plans were compared with optimized conventional wedged plans with
regard to: DVH (breast and lung) and planning time. If the electronic compensator
plans were deemed dosimetrically superior to the conventional wedged plans,
obviously the patients were treated with the electronic compensators.
RESULTS – Especially for large breast and/or asymmetric breasts, the electronic
compensation leads to better target coverage. Also, energy mixing (6MV and 18 MV)
was no longer necessary for many of these cases. Planning time was also
substantially reduced. For small breasts with a single wedge in the medio-lateral
direction, no substantial gain was observed with the electronic compensation.
CONCLUSION – The new modules for the irregular surface compensators (Eclipse,
VMS) allow fast and flexible planning and delivery of a missing tissue compensator
with the dynamic MLC. This is a natural extension of using IMRT tools to improve
dosimetry of conventional 3D plans.
Keywords : electronic/irregular surface compensators; breast treatments; dMLC
52 – MEASUREMENT STRATEGIES FOR SMALL FIELD DOSIMETRY
DURING THE COMMISSIONING OF THE MODULEAF MLC
G. Pittomvils (1), W. De Gersem (1), M. Coghe (1), F. Crop (1,2), B. Van Duyse
(1), F. Jacobs (1), C. De Wagter (1), W. De Neve (1)
(1) Dep.Radiotherapy, Ghent University Hospital, De Pintelaan 185, 9000 Gent
(2) Dep.Medical Physics, Ghent University,Proeftuinstraat 86, 9000 Gent
Intracranial stereotactic radiosurgery treatments are delivered at the U.Z. Gent using
a mini-MLC (Moduleaf MLC) with 40 leaf pairs of 2.5 mm width in the isocenter plane
allowing maximum field sizes of 100x120 mm². The mini-MLC is mounted as an addon collimator on the Elekta SL25 linear accelerator.
A literature study revealed that no ideal detector is available. The diamond detector
has an optimal spatial resolution and negligible energy dependence but is dose rate
dependent. The smallest cylindrical chambers (0.015cc) are limited to field sizes
between 15 mm and 50 mm, have acceptable spatial resolution and no energy nor
dose rate dependence and larger cylindrical chambers (0.125 cc) have limited spatial
resolution.
A large set of measurements was recorded and compared. Profiles were acquired
using the diamond detector, depth dose curves were measured with the pin-point
chamber and output factors were obtained using the three different detectors.
Measurements of a standard field (10x10 cm2) at standard depth (6 MV, 5 cm) for
each detector were used to scale the results to absolute values in cGy/MU. This
enabled the comparison of the points of each percentage depth dose curves to the
central points of the four measured profiles.
Dose rate dependence of the diamond detector was observed in the dose rate range
25-400 cGy/min. The experimental data were fitted using the known theoretical
approximation. After this correction, a good correspondence (average differences <
1%) between the small cylindrical chambers recordings and the diamond detector
recordings is observed for the total set of measured fields, ranging from 15x15 mm²
to 100x120 mm² in size and for measuring depths ranging from 15 mm to 200 mm.
Below that field size an increasing absolute dose difference with decreasing field size
is observed between both detectors probably due to the volume effect of the
cylindrical chambers. The amplitude of this difference however is found to be
insensitive to the measurement depth and therefore relative profile depth
measurements recorded with small cylindrical chambers can be used safely for field
sizes of at least 5x5 mm².
The output factors measured with the three different detectors confirm the
conclusions of the profile, percentage depth dose recordings. The required accuracy
for reproducible output factors using small sized cylindrical chambers was found to
be one tenth of a millimeter for field sizes smaller then 10 mm.
Keywords : small field dosimetry, diamond dectector, pinpoint
53 – CLINICAL INTRODUCTION OF RESPIRATION CORRELATED (RC)
CT/PET
I. Potargent (1), G. Bosmans (2), H. Janssens (1), A. Dekker (2), M. Öllers (2), D.
De Ruysscher (2), P. Lambin (2), A. Minken (2)
(1) Xios Hogeschool Limburg, Diepenbeek, Belgium
(2) Department of Radiation Oncology (MAASTRO), GROW, University Hospital
Maastricht, The Netherlands
PURPOSE – Respiratory motion may reduce the accuracy in imaging lung cancer
patients. Especially with a combined CT/PET scanner differences occurs since the
acquisition time is different for both modalities. The purpose of this study was to
commission and implement respiration correlated software for the PET modality
clinically.
METHOD AND MATERIALS – The Siemens Biograph Sensation 16 CT/PET scanner
was used for the respiration correlated scans. RCPET images were acquired with the
listmode PET software from Siemens and attenuation and scatter correction was
performed with the free breathing CT. Respiration correlated CT (RCCT) images
were retrospectively binned and reconstructed with in house built software. The
respiration signal which was recorded with a pressure sensor in a chest belt (AZ-733
V, Anzai Medical Corporation, Tokyo, Japan) and the software generated a ‘gate’
signal at maximum expiration which was sent to the PET system, this was necessary
for binning the PET images. Both image modalities were binned into ten phases of
the respiration. Phantom testing was performed with a lollipop phantom which was
filled with approximately 20 MBq 18F-FDG. Different movement amplitudes,
frequencies and volumes were imaged to investigate the accuracy of the phase
binning reconstruction for RCCT as well as for RCPET. The first patients underwent
a normal free breathing CT/PET, an RCCT and an RCPET. The volumes and the
movements of the tumor in all these modalities were compared to each other.
RESULTS – With RCCT and RCPET we could accurately measure the shape,
volume and movement of the lollipop phantom within acceptable tolerance level.
Both RC image modalities were capable of determine the average tumor position and
tumor movement in all three orthogonal directions for all patients. There were clear
under and overestimates of the tumor volume between the RC image modalities and
the free breathing CT/PET scan.
CONCLUSION – The RCPET software and the in house built RCCT software were
both able to accurately determine the actual shape of the lollipop phantom. In
patients both modalities could improve the imaging of lung tumors and activity
concentration could be better defined with the RCPET, which will improve automatic
delineation techniques. Further investigation of using phased attenuation correction
still has to be done in patients.
Keywords : respiration correlation, CT/PET, lung cancer
54 – DEVELOPMENT OF A BODY SHAPE CONTOURING SYSTEM
BASED ON THE USE OF A LASER TELEMETER AND DEDICATED TO
PATIENTS TREATED FOR BREAST CANCER .
Van Dycke Michel (1), Duchateau Michel (2), Delmoitié Eric (1), Van Aelst
Ronnie (1), Debusscher Christophe (2)
(1) Clinique Saint Jean, Clinique Saint Jean , Service de Radiothérapie, 32 Bvd du
Jardin Botanique , B-1000 Bruxelles
(2) Altasys (ULB)
For radiotherapy conservative breast treatment more and more irradiation
techniques to assume a homogenous dose repartition in the whole breast without
excessive hot spots are developped. To achieve this objective it is necessary to use
enough information about the shape of the patient but in the same time to garantee
that the position of the patient does'nt change between the simulation or treatment
and the body shape data acquisition.
Due to the special position of the patient on an inclined device it is very often difficult
to match the previous statement on a CT system without to introduce limitations at
the level of the positionning during the simulation. For this reason we have decided to
developp a system able to acquire the patient shape information during the
simulation in the real treatment position. Our system is based on data acquisition of
the patient by a laser telemeter fixed on the simulator head and rotating by 180°
around the patient. The data sended by the laser are captured on a PC and the
developped software transform these data from polar coordinates to cartesian
coordinates and finally export these data directly in the right format to the planning
system. The acquisition time for 1 slice is directly dependent of the speed rotation of
the simulator gantry and takes 27 sec. A whole patient time acquisition for 10 slices
takes about 6 minutes. This application gives also the possibility to enter reference
points and to export the measured data in different formats in order to be compatible
with different treatment planning systems. At the beginning measurements on a
phantom were performed to evaluate the whole precision of the system and both CT
images and external shape acquisition were used for the first 20 patients to compare
results of the 2 modalities. The presentation will cover the differents parts composing
this application : electronic devices , sofware development , formats used and also
the evaluation of the estimated precision of the system.
Keywords : laser telemeter , breast treatment dedicated contouring system
55 – MONITOR CHAMBER BACKSCATTER IN A VARIAN CLINAC
2100C/D LINEAR ACCELERATOR: INFLUENCE ON THE OUTPUT
FACTORS OF A 6MV BEAM
Vanhoutte, Frederik
Heilig-Hartziekenhuis Roeselare-Menen, dienst radiotherapie
Most common systems of dosimetric calculations separate dose, in transient and
lateral electron equilibrium conditions, in several contributions proportional to the
focal energy fluence, taken to be field size independent. Since the fluence itself is an
unwieldy unit in a clinical environment, it is represented by a machine parameter, the
monitor unit, which by definition should also be independent of the downstream
collimator settings of the accelerator. In practice, interaction of the primary photon
beam with the collimators and other structural elements results in backscattered
radiation into the monitor chambers. Depending on the accelator design this can lead
to a doseless contribution to the recorded monitor units, directly affecting the
feedback driven dose rate servo.
This is the case for the Varian 2100C/D Clinac linear accelerator and similar designs.
It uses a single fixed set of monitor chambers for both photon and electron beams.
This necessitates a thin-walled chamber for optimal electron transmission. As a side
effect, the low energy backscattered photons and electrons can easily penetrate.
The amount of backscattered radiation is dependent on the jaw settings, in effect
disturbing the proportionality of the focal fluence and the monitor units.
We investigate the backscatter effect for the 6MV photon beam of this type of linear
accelerator. An open-loop measurement technique is employed in which the dose
rate servo is decoupled from the primary monitor chamber signal. The secondary
dosimetry circuit is modified so the output of the secondary chamber can be directly
recorded with an electrometer. The measured signal decreases by 3.5% ranging
from 0.5cm to 40cm square fields.
However, since the dose rate servo operates on the primary dosimetry circuit, these
results need to be corrected for response differences between the two monitor
chambers. The difference is obtained by comparing the secondary monitor unit
readout with the primary one for different field sizes in the normal operating mode of
the linac. The corrected effect amounts to an additional 2.2% increase in output
factor over the entire range of field sizes.
The overall size-dependence of the effect and the relative contributions of the
different jaws can be reproduced with a simple geometric model correlating the
backscatter with the projected exposed collimator area into the chamber.
Keywords : monitor chamber backscatter, output factor, dosimetry
56 – ICRU 72, HISTORY AND FACTS.
S. Vynckier ( and the other members of the ICRU 72 commission )
UCL, Cliniques univ. St-Luc, Radiotherapy Department
In 1997 a new ICRU commission was established with the task to prepare a report on
dosimetry of beta-ray sources. The report was originally intended to provide guidance
on the specification and calibration of beta-ray sealed sources as used for
Brachytherapy applications. In the beginning the main interest was focussed on
ophthalmic application, however during drafting, the scope of the report was adjusted
to the use of beta-ray sources for intravascular applications due to rapid expansions
of these applications at that time. Moreover, as these beta-ray sources exhibit rather
similar dose distributions as those of low energy photon seed sources and as for a
number of applications the latter are also utilized, corresponding guidance and
recommendations of low energy photon sources was also included in the report. The
report was finally published in 2005.
The presentation will give an overview of the different chapters and will summarize
the recommendations of the report. Moreover, within the scope of the report the
commission members performed also intercomparison measurements and Monte
Carlo calculations for clinical beta-ray sources. The results of these measurements
and calculations will also be presented.
Keywords : ICRU, brachytherapy, beta- and low energy photon sources
57 – TESTING OF THE ANALYTICAL ANISOTROPIC ALGORITHM (AAA)
FOR PHOTON DOSE DISTRIBUTION
Martin Morelle (1), Ann Van Esch (2), Hannu Helminen (3), Laura Tillikainen (3),
Sami Siljamäki (3), Dominique P. Huyskens (2)
(1) Clinique Ste-Elisabeth Namur, Radiotherapy Department, Belgium
(2) 7Sigma, Belgium
(3) Varian Medical Systems, Finland
PURPOSE – To test the accuracy of the new algorithm AAA implemented in the TPS
Eclipse (Varian Medical Systems). AAA was developed to replace the single pencil
beam (SPB) algorithm for photon dose distributions and to improve the dose
calculation accuracy in heterogenous media. Tests were performed for 6 MV and 18
MV photon beams (Clinac 21EX VMS)
MATERIAL & METHODS – The AAA algorithm consists of two modules: a
configuration module and a dose calculation engine. The configuration module: this
part of the program constructs the phase space of the treatment unit by optimizing
the agreement between the calculated and measured depth dose curves and
profiles. The phase space is described by a primary photon source, a secondary
finite-size photon source and an electron contamination source. The dose calculation
engine: the dose in the patient is calculated as the superposition of the dose
deposited by the two photon sources and by the electron contamination source. The
photon dose is calculated by a 3D convolution of Monte-Carlo pre-calculated scatter
kernels. The interacting scatter kernels are scaled using the electron density matrix.
The accuracy of AAA was tested for different field sizes and under the following
conditions: open fields and MLC fields (depth dose curves, profiles, monitor units);
IMRT fields (depth dose curves and profiles); cork inhomogeneities (depth dose
curves and profiles). An intercomparison was made between measurements, AAA
calculations and SPB calculations.
RESULTS/DISCUSSION – The calculated phase space reconstructs the depth dose
curves, profiles and monitor units of open beams with good accuracy. Especially the
second photon source improves the penumbra modeling. Depth dose curves of MLC
fields and IMRT fields are adequately modeled. Depth dose curves in the cork are
substantially better with AAA than with the single pencil beam, but not in the solid
water behind the cork. Profiles at the interface between solid water and cork show in
general a better agreement with AAA.
CONCLUSION – Compared to SPB, AAA improved in most cases the accuracy of
dose calculations. Splitting up the algorithm in a configuration module and a
calculation engine offers the possibility to progressively fine-tune the calculation
engine.
Keywords : TPS testing, photon dose algorithms, phase space
58 – EVALUATION AND VALIDATION OF GYN GEC-ESTRO
RECOMMENDED TARGET CONCEPTS AND DOSE VOLUME
PARAMETERS OF MR BASED BRACHYTHERAPY FOR CERVIX
CANCER.
A. Nulens (1), S. Lang (2), E. Briot (3), M. De Brabandere (1), C. Kirisits (2), J.
Dimopoulos (2), C. Haie-Meder (3), R. Pötter (2), E. Van Limbergen (1)
(1) University Hospital Gasthuisberg, Leuven, Belgium
(2) Medical University of Vienna, Vienna, Austria.
(3) Institut Gustave Roussy, Villejuif, France
The GYN GEC-ESTRO group has recently defined recommendations for target
delineation (Haie-Meder et al.) and reporting 3D image based brachytherapy of
cervical cancer (Pötter et al.). The recommended concepts and parameters were
validated during delineation workshops carried out in the 3 participating centres. In
this study we present data of 6 patients, 2 patients from each centre with good and
poor remission after external beam therapy and before brachytherapy.
METHODS – For each patient 3 target volumes GTV, High Risk CTV (HR CTV) and
Intermediate Risk CTV (IR CTV) were delineated on axial MR images at time of
brachytherapy with the applicator in place. Contouring was performed independently
by 3 physicians of the participating centres according to the target delineation
protocol defined by our group. The patients received a brachytherapy plan according
to the treatment approach of the hospital of actual treatment. Treatment approaches
differ in dose rate (HDR,LDR,PDR), type of applicators, dose specification methods
and dose prescription. Absolute values and localisation of target volumes were
compared among the 3 physicians, as well as dose volume parameters D100 and
D90. In order to compare doses of different dose rate and fractionation schedules, all
doses were biologically normalized to 2Gy per fraction external beam therapy
(EQD2) equivalent to 50cGy/h LDR using the linear quadratic model of
monoexponential sublethal damage repair.
RESULTS – The average standard deviation in percent for the 6 patients with regard
to the average volume within each patient was 48.7%, 20.3% and 14.1% for GTV,
HR CTV and IR CTV respectively. Differences in absolute volumes and localisation
of target volumes were due to differences in interpretation of radiological images of
the anatomy, differences in integration and translation of clinical information onto MR
images, inherent differences present in the delineation process itself and specifically
for GTV due to the very small absolute volume at time of brachytherapy. The average
standard deviation in percent for the 6 patients with regard to average doses D100
and D90 within each patient was less then 5% for HR CTV and IR CTV. Dose
differences for GTV were higher (average SD 11%) due to larger differences in
absolute target volume.
CONCLUSION – Definitions of the target concepts as recommended by the GYN
GEC-ESTRO group proved to be unequivocally applicable and clear in a small group
of three experienced institutions.
Keywords : cervix brachytherapy, target concept, dose volume parameters
59 – LEAF AND JAW POSITION OPTIMIZATION FOR INTENSITYMODULATED ARC THERAPY (IMAT) APPLIED FOR A RECTAL CANCER
CASE
Luiza A.M. Olteanu, Ir.,Werner De Gersem, Ir., Wim Duthoy, M.D., Marc Coghe,
Lic., Wilfried De Neve, M.D., Ph.D.
Division of Radiotherapy, Ghent University Hospital, Ghent, Belgium
PURPOSE – Our in-house developed optimization software [1] was adapted in order
to take into account the dynamic constraints imposed by the delivery of the IntensityModulated Arc Therapy (IMAT, [2]) plan on an Elekta delivery system. The
optimization of the X jaw positions was included in the optimization software along
with a technique that handles the speed constraints of X jaws and multileaf collimator
leaves. The purpose of this presentation is to show the advantages and
disadvantages of these adaptations.
METHODS AND MATERIALS – The IMAT optimization cycle is divided into three
parts: (1) monitor unit optimization, (2) leaf and jaw position optimization and (3) leaf
and jaw position adaptation according to their speed constraints (there is no
optimization made in this part). This cycle is repeated for a predefined number of
times and the quality of the plan is reflected in the value of the biophysical objective
function [1]. We applied this procedure on a rectal cancer case for which the beam
shapes were collimated around the Planning Target Volume (PTV). Hereafter we
expanded the PTV volume with 1 cm and three plans were made. For one plan, the
jaw positions were set manually at 8 mm from the expanded PTV and leaf positions
were optimized. For the other two plans, the beam shapes were left unchanged and
we optimized one plan with automatic jaw position optimization and the other without.
The intermediate values of the objective function during the optimization were
recorded. The final Dose-Volume Histograms (DVHs) of all targets and critical organs
were computed after all optimizations.
RESULTS – At the end of some of the optimization cycles, the value of the objective
function became worse after the application of speed constraints; however this
decrease in the quality of the plan was compensated by the subsequent optimization
cycles. There were no important differences between the DVHs of the critical organs
obtained after all three optimizations. The partial volume of the PTV that was
underdosed, was smaller for both plans in which the jaw positions were changed.
CONCLUSION – Manual adaptation of jaw positions could be replaced by their
optimization implemented inside the optimization software. The final positions of both
leaves and jaws met the speed constraints imposed by the delivery system.
[1] De Gersem W. et al. IJROBP 2001; 51; 5; 1371-1388
[2] Duthoy W. et al. IJROBP 2004; 60; 3; 794-806
Keywords : IMAT, leaf and jaw position optimization, rectal cancer
60 – MODELLING THE MODULEAF MLC IN PINNACLE
G. Pittomvils (1), M. Coghe (1), W. De Gersem (1), F. Crop (1,2), B. Van Duyse
(1), F. Jacobs (1), C. De Wagter (1), W. De Neve (1)
(1) Dep.Radiotherapy, Ghent University Hospital, De Pintelaan 185, 9000 Gent
(2) Dep.Medical Physics, Ghent University,Proeftuinstraat 86, 9000 Gent
The stereotactic module of Pinnacle 7.4 does not support a mini MLC. The standard
module was used to implement the moduleaf MLC of MRC for treatment planning of
stereotatic intracranial applications. The auto-modelling software however proved to
be unsuitable for small fields below 20x20 mm² and differences of more then 10 %
were observed between modelling and measured data especially for the depth dose
profiles.
Manual modelling was started, using the percentage depth dose curves of field sizes
of 10x10 mm² and 20x20 mm² instead of the standard 100x100 mm² field size.
Using a manual fit of the energy spectrum, source size, Gaussian height and
Gaussian width and the off axis softening factor a good agreement between the
measured and calculated profiles and depth dose curves was obtained for field sizes
from 5x5 mm² to 100x100 mm².
The leaf-tips are modelled using the rounded leaf end model available in the
software. The checking of this manual beam modelling was done on different levels
1. The calculated output measurements were checked for square fields ranging
from 5x5 mm² to 100x100 mm² using the finest applicable calculation grid
(0.1mm).
2. Half beams parallel and orthogonal to the leaf settings were compared with
measured data at four different depths 15, 50, 100 and 200 mm.
3. Odd shaped fields using planes with hinge angles of 30,45 and 60 degrees to
the leaf orientation were used to check the penumbra shaping of Pinnacle 7.4
at the same four depths.
4. Finally the penumbra in the edge of an angle of 105 degrees is checked to
the measured data at the same four depths.
The obtained results are within the acceptance levels for single fraction stereotactic
treatments.
1. A maximal deviation of 0.5 % on the output factors is observed.
2. For standard field settings, a maximal deviation of 1 mm between the
measurements and the calculations in the field profiles is observed.
3. Only for a 60 degree plane, a deviation exceeding 1 mm but inferior to 2 mm
is recorded around the 20 % and 90 % isodoses.
The Collapsed Cone algorithm used by the Pinnacle planning system is well adapted
for the modelling of field sizes used for stereotactic treatments when a separate
machine is modelled for those small field sizes using a 10x10 mm² or 20x20 mm² as
the standard field size for modelling the machine parameters.
Keywords : Pinnacle, modelling, stereotactic dosimetry
61 – QUALITY ASSURANCE IN RADIOTHERAPY BY A MAILED
THERMOLUMINESCENCE DOSIMETRY PROCEDURE AT THE PATIENT
LEVEL
Ans Swinnen, Jan Van Dam, Walter Van den Bogaert
UZ Gasthuisberg
PURPOSE – During transfer of large amounts of information between the steps in
the radiotherapy process, errors can occur that can result in systematic or random
errors in individuals or in a group of patients. QA programs are therefore needed and
should not only include regular internal checks performed by the centres’ physicists,
but also external audits made by an independent organisation. Latter programs
relying on postal dosimetry systems are generally limited to beam output checks by
thermoluminescence dosimeters (TLDs) in a water phantom. Phantom
measurements however do not reflect by definition the total accuracy of patient
treatment delivery. Hence, in vivo dosimetry should be a complementary part of the
QA methods to assess the overall accuracy from prescription to delivery.
METHODS – For application of TLDs in mailed in vivo entrance dosimetry studies,
cylindrical build-up caps in Al, Cu and Ta are developed with thickness
corresponding to the depth of maximum dose for photon energies between
respectively 60Co gamma rays, 4-10MV and 10-18 MV X-rays. The dose attenuation
by the detectors (i.e. TLD in build-up cap) is investigated by film dosimetry. Next, the
feasibility of the detectors is verified by placing them on patients treated in the
Leuven radiotherapy department. Further, the detectors are mailed to 22 European
radiotherapy centres that select a cohort of patients and calculate the entrance doses
by their TPS.
RESULTS – Phantom measurements reveal that the caps have an adequate shape
and thickness. The detectors show a clinically acceptable local perturbation. Two in
vivo measurements per patient demonstrate that the reproducibility of the dose
measurement is on average better than 2%. A tolerance level of ±5% is applied. The
mean ratio of measured to calculated entrance doses for the pilot in vivo study is
0.994±0.021(N=26). For the mailed in vivo study, the mean ratio is
0.999±0.087(N=216). Some significant systematic errors are traced. The majority of
discrepancies between measured and calculated entrance doses are random
variations in patient setup. An adjusted tolerance level for certain clinical indications
could be desirable, for instance for breast cancer patients with wedged treatment
fields.
CONCLUSION – We have been successful in our attempt to provide a validated
methodology which extends the basic mailed dosimetry audits to the level of patients
with the more clinically related checks by mailed in vivo dosimetry with TLDs
Keywords : thermoluminescence dosimetry, mailed, in vivo
62 – TOTAL BODY IRRADIATION: THE MIDDELHEIM PARADIGM
J. Vanregemorter, P. Meijnders, D. Van den Weyngaert
ZNA Middelheim hospital, Department of Radiation Oncology
Lindendreef 1, B2020 Antwerp, Belgium
PURPOSE – In this study we describe a technique used for fractionated total body
irradiation. The patient is treated with 12 Gy in 6 fractions over 72 hours with the
dose to the lungs limited to 9 Gy.
METHODS AND MATERIALS – After comparison of several techniques for
positioning the patient (antero-posterior irradiation with patient laying on his side, free
standing, sitting or crouching, left-right irradiation with patient in supine position) and
positioning the lung shields, we have opted for a standing position of the patient, fully
stretched with adjustable crouch and armpit supports and hand grips. A reclining
back support at 15 degrees gives the effect of a resting position. The ”treatment
cabin” is positioned at 3 meters from the linac’s isocenter. For the lung shields we
use transmission blocks cut out of lead sheets. The blocks are glued on a plastic foil
which is fixed by two clamps to the 2 cm thick PMMA screen used to homogenise the
surface dose. To track the position of the blocks and make it reproducible, during
simulation a 8mm steel ball bearing is put on the patient on the projection of a central
mark on the PMMA screen. This mark (or rather the ball bearing) shows up on an “xray” taken with 12 MU (18 MeV) on a CR screen. The magnification known, a
reduced copy is printed to give the size of the lung blocks and their position relative
to the central mark. Once the blocks are cut, they are glued on the foil relative to the
mark. Positioning the mark on the foil onto the mark on the PMMA screen gives a
reliable reproducible position of the blocks relative to the patient. During treatment,
each session starts with a verification “x-ray” of the blocks and the lungs.
RESULTS – Only small (1-4 mm) adjustments of the position of the block foil are
necessary depending on the general status of the patient. The reclined standing
position combined with the support and individual head rest gives a very reproducible
position for both anterior and posterior irradiation. Patients experience the position as
rather comfortable.
MEASUREMENTS AND CALCULATIONS – Depth dose and transmission were
measured at the effective distance of the treatment. Lung dose relative to central
dose is measured on line by diodes every treatment fraction. TLD measurements
track central dose and dose to extremities.
CONCLUSION – After a series of 15 patients we can conclude that the described
paradigm allows for routine TBI treatment. The specially designed treatment cabin is
easy
Keywords : total body irradiation
63 - STUDY RADIOLOGICAL PHYSICS ON DENTAL CONES
Jean-Louis Greffe
CHU de Charleroi
This study makes it possible to check the validity of the standards of the documents
RP91, in particular from the point of view:







output of the tube with X-ray (microGy/mAs à 1 m).
reproducibility of the output of the tube with X-ray.
precision of the tension of the tube with X-ray.
reproducibility of the tension of the tube with X-ray.
precision of time of the irradiation.
reproducibility of time of the irradiation.
measurement of the layer of half attenuation
Conditions of measurements :




numbers of tube with X-ray
33
type of the device
dental cone
measuring apparatus
Baracuda
probe detection
MPD
Preliminary results

output of the tube with X-ray (microGy/mAs à 1 m), for tensions of 50 to 70 kV.
o 42,0 (sd = 54,4 %), the norm 30<x<80.
o tube with X except limit, 2 dental cones with results too light and 1 dental
cone with results too high.

tension of the tube with X-ray, variation with the nominal voltage.
o - 4,6 (sd = 107 %), the norm <10%.
o tube with X except limit, 4 dental cones with results too light.

time of the irradiation (%), variation at the irradiation time.
o 1 (sd = 1700 %), the norm <10%.
o tube with X except limit, 1 dental cones with results too light and 6 dental
cone with results too high.
Conclusion
There can be strong variation so much from the point of view of precision of the
tension of the tube (tension too weak) that precision of the timer (irradiation time too
high).
64 - ...
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65 – AUTOMATED TUMOR DELINEATION IN FDG PET IMAGES
F. Jacobs, B. Vanderstraeten, W. De Gersem, G. Pittomvils, M. Coghe, B. Van
Duyse, C. De Wagter, W. De Neve
Dep.Radiotherapy, Ghent University Hospital, De Pintelaan 185, 9000 Gent
PURPOSE – Current research on the automated delineation of FDG PET tumours is
usually based on thresholds, where the authors try to optimize this threshold based
on the loco-regional signal-to-noise ratio and recovery coefficients. Such methods
require the acquisition of calibration curves using phantoms. We present an algorithm
based on a gradient filter, which does not use a threshold, and does not require
measurements. We investigated the effect of the applied pixel neighbourhood –
usually not mentioned in any article – and the applied gradient filter on the derived
tumour volume.
METHODS – The PET image is first filtered with a low pass average filter to reduce
image noise. Then a gradient filter is applied which replaces each pixel value by the
local slope. Finally a region growing is applied on the gradient image using a
predefined pixel neighbourhood, starting from a predefined seed point. Region
growing is performed until each adjacent pixel holds a smaller slope. The maximum
pixel value in the delineated region is determined and used as the new seeding point.
The process stops when the seeding point becomes constant. The volumes of
sixteen head and neck tumours have been determined. Three gradient kernels (G1,
G2, G3) and three pixel neighbourhoods (6, 18, 26 pixels) were investigated.
Differences were interpret with a mixed model ANOVA technique, with the gradient
kernel and the neighbourhood as fixed factors and the patient number as a random
factor. Post-hoc Scheffé tests were performed for gradient kernel and pixel
neighbourhood. No cross products were taken into account.
RESULTS – All tumours were delineated without problems. Results were
independent of the initial seeding point, as long as this point was not too far downhill.
No significant differences were found with respect to the applied gradient (p = 0.939).
The pixel neighbourhood, however, did have a significant influence (p < 0.001). After
visual inspection, we decided to use a neighbourhood of 18 pixels for future studies.
Aside from the 16 investigated tumours, we also investigated the robustness of this
method for other types of tumours and other locations. All investigated tumours
rendered robust results, i.e., independent of the initial seeding point and not “running
away” into regions with high uptake, e.g., the brain or the heart.
CONCLUSION – An algorithm for automated tumour delineation in FDG PET images
has been proposed. The algorithm yields robust results. The applied pixel
neighbourhood has a significant influence on the derived volumes.
Keywords : delineation, FDG, PET
66 – Voxel phantom ...
Jessica Pages, Filip Vanhavere