Download Accuracy Requirements in Radiotherapy Treatment Planning

REVIEW ARTICLE
Accuracy Requirements in Radiotherapy Treatment Planning
Saeed Ahmad Buzdar1,2, Muhammad Afzal2, Aalia Nazir2 and Muhammad Asghar Gadhi3
ABSTRACT
Radiation therapy attempts to deliver ionizing radiation to the tumour and can improve the survival chances and/or quality
of life of patients. There are chances of errors and uncertainties in the entire process of radiotherapy that may affect the
accuracy and precision of treatment management and decrease degree of conformation. All expected inaccuracies, like
radiation dose determination, volume calculation, complete evaluation of the full extent of the tumour, biological behaviour
of specific tumour types, organ motion during radiotherapy, imaging, biological/molecular uncertainties, sub-clinical
diseases, microscopic spread of the disease, uncertainty in normal tissue responses and radiation morbidity need sound
appreciation. Conformity can be increased by reduction of such inaccuracies. With the yearly increase in computing speed
and advancement in other technologies the future will provide the opportunity to optimize a greater number of variables
and reduce the errors in the treatment planning process. In multi-disciplined task of radiotherapy, efforts are needed to
overcome the errors and uncertainty, not only by the physicists but also by radiologists, pathologists and oncologists to
reduce molecular and biological uncertainties. The radiation therapy physics is advancing towards an optimal goal that is
definitely to improve accuracy where necessary and to reduce uncertainty where possible.
Key words:
Radiotherapy. Treatment planning. Accuracy requirements. Errors. Uncertainties.
INTRODUCTION
In recent years, there have been significant improvements in the field of radiotherapy. Advances in computer
hardware and software, and medical imaging have led to
the development of new technology for improving
external beam treatment planning, dose delivery and
verification or radiotherapy.1 The principle of treatment
planning is based on the attempt to arrange the
treatment field so as to conform the high dose region to
the target. It also attempts to minimize the radiation dose
to normal structures and constrain the dose to critical
structures below tolerance. Poor local tumour control or
increased normal tissue complications may arise from
inaccurate targeting of the tumour, failure to conform the
high dose distribution to the target volumes and
inaccurately delivered radiation doses.
High radiation doses can cause severe health complications or death if delivered to the normal tissues, but
these are critical and sometime necessarily needed to
be given to a patient for his survival. Delivery of high
radiation dose to the tumour without injuring proximal
normal tissues requires accuracy and precision in the
1
2
3
Department of Medical Physics and Bioengineering, University
College London, London, UK.
Department of Physics, The Islamia University of Bahawalpur,
Bahawalpur.
Department of Radiation Oncology, Bahawalpur Institute of
Nuclear Medicine and Oncology (BINO), Bahawalpur.
Correspondence: Dr. Saeed Ahmad Buzdar, Assistant Professor,
Department of Physics, The Islamia University of Bahawalpur,
Bahawalpur.
E-mail: [email protected]
Received: May 09, 2012; Accepted: February 25, 2013.
418
delivery of radiation to the tumour. Treatment outcome
can be improved by improving accuracy and precision in
delivering dose to the conformed target with the help
of recently developed three dimensional imaging
techniques. Confirmation of the accuracy of optimized
calculations with verification evaluation techniques is
vital in order to assure the quality of treatment.2
There are different possible errors and uncertainties that
may exist in different areas in radiotherapy treatment of
a patient. These errors need to be identified, and then
removed where possible.3 The present review discusses
in detail about the possible errors and uncertainties in
radiotherapy treatment planning, by underscoring the
significant areas in accuracy point of view, which broadly
are accuracy of immobilization, organ movement,
imaging, definition of target volumes, choice of dose
distribution, treatment verification, machine checks, and
Dosimetry protocols, planning checks, patient documentation and treatment scheduling.
Target volume determination: Defining the target volume
is one of the important steps in treatment planning.
Radiation oncologists currently define Planning Target
Volume (PTV) empirically. Different problems of tumour
localization and geometrical reconstruction has been
investigated and solved in research works done
previously.4,5 For accurate dose reporting, the volume of
interest description is a pre-requisite for meaningful 2-D
and 3-D treatment plans.6
These regions of interests, target and normal structure,
delineation should follow the International Commission
on Radiation Units and Measurements (ICRU) -50 and
62 Reports. Volumes of interests for target delineation
started from grass tumour volume to planning target
Journal of the College of Physicians and Surgeons Pakistan 2013, Vol. 23 (6): 418-423
Accuracy requirements in radiotherapy treatment planning
volume. A radiation oncologist usually determines these
anatomical clinical volumes, often after other relevant
specialists such as pathologists or radiologists have
been consulted. These entire volumes required different
dose levels around there self for the best achievement of
goal of radiation therapy.7
In some cases Grass Tumour Volume (GTV) and Clinical
Target Volume (CTV) are considered as same volume
but in other cases these volumes usually differ by
different margins as GTV plus very small cm values for
regular or irregular GTV.8
To determine the target volume, any palpable masses
are marked with wire and contrast medium may be
placed in the bladder, rectum, vagina or esophagus.9
The most difficult and subjective step in treatment
planning is to quantify margins around grass tumour
volume to define clinical target volume.10 It needs a lot
of practice, clinical experience and the knowledge of
the patterns of tumour recurrence so that precise data
be used to define the margins around GTV. Different
factors like age of the patient and considerations of
normal tissue tolerance, may influence the maximum
volume considered to be appropriate for treatment.
The concepts of defining target volumes are also
based on the risk of microscopic spread of the disease.8
There is a chance of error in defining PTV by different
oncologists, and sometimes even by the same
oncologist on different occasions.11,12 National Cancer
Institute (NCI) photon treatment group made efforts to
standardize the process of defining PTV13 and
international commission on radiological Units14 resulted
in the definition of the PTV in relationship to the GTV and
the clinical target volume (CTV).
Organ at risk (OARs) and normal tissue tolerance:
Tumours arising from different regions of the body vary
in pathology, size, shape etc. Added to the fact that
there may be different pattern of Organs at Risk
(OARs) present in close proximity, a variation of the
level of treatment planning difficulties and complexity
is expected. As a consequence, the degree of dose
conformity to the PTV would also be affected, where
certain tumours are easier to conform than the others.15
Normal tissues are also irradiated along with the tumour
but there is wide variation in the intrinsic radio sensitivity
of different normal tissues and susceptibility to changes
in fraction size. Therefore, the site and volume of normal
tissues must be defined clearly. Increased shielding of
parts of normal organs is now feasible with the use
of conformal blocks and multileaf collimators, with
calculation of dose volume histograms for description of
normal tissue doses.9 The treatment plans should be
evaluated with reliable calculation systems for accurate
and optimized outcome.16
Some of the organs have very small tolerance dose
value for example lenses of eye are organs at risk during
nasopharyngeal or brain tumour treatments as according
to the American Center of Radiology, the lens of eye has
just 1200 cGy tolerance dose which causes 50% injury
within 5 years. In complete treatment planning, which
consists of almost 32 fractions, the dose per fraction and
total dose should be considered.17
Organ movement: Modern radiotherapy techniques
intend to focus on the motion of target volumes, to
guarantee the higher degree of accuracy. The treatment
of lung cancer with external beam therapy presents a
challenge due to the existence of breathing motion
during both the simulation and treatment. The effects
of normal breathing on coverage are small on the
average, with a less than 4% chance of a 10% or greater
decrease in Tumour Control Probability (TCP). 18
However, in patients with large respiration-induced
motion, the effect can be significant and efforts to
identify such patients are important.
Stereotactic body frames are suggested to be used for
real-time tumour-tracking, for an effective reduction in
respiratory intrafractional organ motion.19
Dose distribution and the degree of conformation:
The geometry of the radiation fields can be defined by
Three Dimensional (3-D) information in conformal radiotherapy process and then dose distribution is optimized
using the Beam's Eye View (BEV) concept. The BEV
display shows the anatomy from the perspective of a
viewer positioned at the beam's source, looking at the
patient along the central axis of the beam.20
Conventional dose calculation formalism, such as
radiological path length and scatter integration models,
is not sufficiently accurate for three dimensional
conformal radiotherapy (3DCRT). The main weakness of
conventional dose calculation is the inability to calculate
dose accurately in regions of electronic disequilibria,
such as under or near boundaries of arbitrary shaped
beam defining devices.21
Sometimes dose distribution is not as it is needed, then
it can be altered by using wedges so that it will
compensate missing tissues as well as produce a more
homogenous result.
For the treatment of deep seated tumours, skin and
other superficial tissues are spared by combination of
several beams. Computerized dose planning systems
are used to construct an isodose distribution with
beams of appropriate energy, size, weighting, gantry
angle and wedge to give a homogenous result over
the target volume. The combination of the beams in
order to concentrate the dose at the target volume can
be performed according to different methods. noncoplanar beams can be useful to spare critical organs,
especially when high level of dose is required.22-24
Conformity Number (CN) has been used first to quantify
the degree of conformity in radiotherapy of prostate
Journal of the College of Physicians and Surgeons Pakistan 2013, Vol. 23 (6): 418-423
419
Saeed Ahmad Buzdar, Muhammad Afzal, Aalia Nazir and Muhammad Asghar Gadhi
cancers by external beam and interstitial implantation25
and then concept of Conformity Index (CI) was
introduced in different documents.7,15,26
Computed Tomography (CT) and Magnetic Resonance
Imaging (MRI) provide a 3-D model of the patient's
anatomy and tumour, which allows radiation oncologists
to more accurately prescribe irradiation to the tumour
while sparing neighboring critical normal organs.27
Dose calculation systems: Diverse calculation
algorithms are integrated in commercial treatment
planning systems, to calculate the dose distribution in
target volumes and organs at risk. Each target volume
along with its surrounding healthy tissues requires
specific dose distribution so that the beam capable of
providing that certain distribution and having a behaviour
to traverse desired dose delivery should always be
selected for radiotherapy.28
The reliability evaluation of calculation systems is vital
otherwise they may mislead to an inappropriate set of
planning parameters. In a comparison made between
pencil beam and collapsed cone calculation system, it
was found that both do not differ significantly in central
axis fields as well as large field sizes, however, in the
case of off-axis fields and small fields, there is notable
variation.29 Figure 1 demonstrates this deviation of the
collapsed cone calculation system.
Figure 1: Percentage depth dose curve of 6 MV photon, for 2x2 cm2 off axis
corner field, at depth of 10 cm.16
Beam shape: Appropriate beam shaping can be made
possible with the help of low-melting point alloy blocks
and multileaf collimators. Conformal radiotherapy refers
to a method of treatment delivery that incorporates rigid
immobilization and 3-dimensional computer planning
and treatment systems to produce a high dose area of
radiation that conforms to the shape of the target.
Enhanced conformation allows for greater dosage of
radiation to reach the target volume while delivering less
radiation to surrounding normal tissues. Conformal
therapy uses a number of different techniques to shape
the beam and miss normal tissues. The simplest
420
technique is called multi field, static coplanar irradiation.
That is, the multiple converging beams of radiation lie in
a single plane.30
In conformal radiotherapy treatment planning, it is aimed
to enhance Tumour Control Probability (TCP) by
improving the spatial distribution of relative dose.
Reducing normal tissue irradiation should be at
minimum level and consequently increasing the
prescribed dose should be preferred until normal tissue
tolerance is reached.31
Actually, the determination of the Tumour Control
Probability (TCP) and of Normal Tissue Complication
Probability (NTCP) permits to summarize with a few
significant figures the potential outcome of a complex
3-D dose distribution.32-34
The shape of the dose distribution and the value of
isodose can be affected by the attenuation of X-ray
beam by relative changes in tissue densities. So,
inhomogeneity corrections are introduced. A pixel by
pixel correction can be made to take account of all tissue
densities within the body contour by conversion of CT
numbers into relative electron densities.
Dimensions of treatment field are defined from external
or radiological anatomical landmarks, assuming
implicitly that the PTV receives adequate coverage.
Simulator films and Beam Eye View (BEV) approach are
used to shape the field. BEV approach which is
considered superior in order to design better edges of
the fields.35 It requires that PTV is previously acquired
and reconstructed by the computer.36-39
Imaging: After completion and approving the treatment
plan, the CT image set is used to obtain Digitally
Reconstructed Radiographs (DRRs) that are computer
generated projection images produced by mathematically passing divergent rays through a CT data set and
acquiring X-ray attenuation information along the rays
during 3-D conformal radiotherapy treatment planning.
They are essential for implementing 3-D CRT. The DRR
serve as a reference image for transferring the 3-D
treatment plan to the clinical setting. Its role is similar to
that of a simulation film.40
It should be recognized that the planning procedures is
based on 'snap-shot' image data and there can be a
significant delay between the CT session and when the
images were acquired and the start of the actual
treatment. Consistency between the planning set-up and
the treatment set-up on a daily basis is facilitated by use
of immobilization and verification devices. Verification
can include comparison of portal image data with the
planning or simulation images.41 The best possible
imaging for site of interest should be used, as it has
been reported that MR images can reduce the spatial
uncertainty for prostate treatments. 42 Ultrasound
imaging can also be used for effective use to localize the
prostate motion.43
Journal of the College of Physicians and Surgeons Pakistan 2013, Vol. 23 (6): 418-423
Accuracy requirements in radiotherapy treatment planning
Documentation and transfer of patient data: All the
treatment parameters of a patient like number of fields,
machine settings, monitor units and other details are
stored first and then transferred to the treatment
machine. This is done at the time of treatment. This all
should be performed carefully and any update in detail
of the patient's stored parameters should be transferred
too, to the machine. Medical physicist is responsible to
create treatment plan and to send it to machine via
network. Modern machines have the option that
technologists cannot change any parameter without the
permission of medical physicist.44
Machine checks: It has been shown that 3DCRT
plans are more sensitive to deliver errors and, therefore,
it becomes more important to verify that 3DCRT
treatments are delivered correctly. Record and Verify
(R and V) system can be used to check that machine
parameters have been entered correctly. Machine
parameters that can be verified by R and V system are
gantry and collimator angles, field size, beam energy
and type, monitor units and accessories such as blocks
and wedges. Correct patient position should be checked
by port films or electronic portal images. This position
record should be compared with DRRs.
Patient positioning and immobilization: The issue of
patient positioning was discussed in documents45-47 and
all available data support the use of an immobilization
device in order to achieve stable and reproducible
patient positioning during treatment planning and
delivery. The clinical aspects of treatment simulation rely
on the positioning and immobilization of the patient as
well as on the data acquisition and beam geometry
determination.
It is important that the patient be treated in one position
only, as changes may result in alterations in internal and
external anatomy and risk of over or under dosage.
Taking a decision on patient position and ensuring that
this position is reproducible throughout the whole
procedure is the first job to do in patient data acquisition.
Specific immobilization systems are to be used for a
conformal treatment. In brain treatment and possibly
other localizations, stereotactic frames allow to reach a
better accuracy.48
The position of the patient, all positioning aids and
anatomical measurements should be documented
accurately in writing with a diagram and Polaroid
photograph to ensure reproducibility through all stages
of the planning process and subsequent treatment.
Localization of the target volume within the patient in
relation to external reference points should be done
under exactly the same conditions as subsequent
treatments.
If treatment will use a beam coming posteriorly through
the couch, a 'tennis-racket' insert or other window in
the couch may be considered to avoid attenuation
of the beam. This can lead to inaccuracy of patient
Figure 2: Hybrid head mask for immobilization and positioning.
positioning, and a rigid carbon-fiber insert may be used
instead. If any immobilization system has been used for
patient data acquisition, it must also be used for
treatment verification and execution. The Figure 2
shows reinforced thermoplastic masks which add an
unparalleled horizontal stability with an increased
fixation force.
The protected rim around the mask and over the chin
and shoulders results in great control over the actual
position of the patient positioning and patients head.
Effect of the setup errors in radiotherapy treatment:
Although there are a number of studies which attempted
to assess the magnitude of setup errors, analysis of their
dosimetric effects is quite sparse. In an analysis of setup
errors in 3-D conformal plans, it could be found more
sensitive to setup errors than traditional plans. To
measure setup errors, two double exposure portal films,
approximately 90° apart, should be taken before each
fraction of the patient. Five to seven landmarks should
be identified on each film, and the translation and
rotation of the patient should be calculated using a least
squares algorithm. For each fraction, three dimensional
dose distributions should be calculated for each fraction
using the measured position of radiation isocenter
relative to the patient.
One striking feature is that the nominal plan is an overly
optimistic estimate of the intended treatment. As
indicated by the range of daily Dose Volume Histograms
(DVH) and the mean DVH, almost all daily treatments
indicate less of the target at each dose level than the
planned treatment. This result appears to be a
consequence of the fact that almost any setup error
leads to a decrease in dose to the target since the dose
distribution in this case surrounds it conformally. In
contrast, if the patient were treated with a traditional
parallel opposed plan the range of daily dose distributions would be closer to the nominal or intended plan.
Similar results and conclusions follow when DVHs for
normal organs are analyzed.
Journal of the College of Physicians and Surgeons Pakistan 2013, Vol. 23 (6): 418-423
421
Saeed Ahmad Buzdar, Muhammad Afzal, Aalia Nazir and Muhammad Asghar Gadhi
Incorporation of uncertainties in treatment plan:
While online imaging in combination with protocols to
correct setup errors may reduce their magnitude, it is
evident that the residual systematic and random errors
will still remain. Moreover, organ motion will affect the
accuracy of dose delivery and these motions may be
difficult or impossible to control. One approach, and the
standard one, is to understand the errors inherent in a
treatment site and then to apply margins around the
CTV, which will account for these treatment
uncertainties. Another approach is to directly include the
uncertainties in the dose calculation algorithm. 49
Conformal treatment aims to conform high dose volume,
usually the prescribed dose volume; tightly to the
planning target volume and spare the organs at risk in
close proximity. Recent studies reported that 3DCRT,
Stereotactic Radiotherapy (SRT) and Intensity
Modulated Radiation Therapy (IMRT) have the potential
for tumour dose escalation, better tumour control,50-52 as
well as have the ability to compensate the systematic
uncertainties by including multi-leaf collimator margins
for better clinical outcomes.53
CONCLUSION
There have been great improvements in radiotherapy
techniques. Significant changes in the outcome of the
radiation therapy treatment have been noted in recent
years. A lot of work has been done by the physicists and
oncologists to improve accuracy in the entire process of
radiotherapy treatment. Improved dose calculation
methods are available and their use is necessary to
remove dose uncertainty as a factor in determining
outcome. However, improved accuracy is usually
accompanied by a reduction in the speed of calculation.
It is important that the speed of calculation is fast
enough for the interactive development of treatment
plans. The objective is to determine what approximation
can be made to enable the accurate calculation of dose,
in a reasonable time and affordable system.
It is necessary for different techniques, to perform a
systematic analysis of the set-up uncertainties and to
translate them into 3-D margins to be applied around the
CTV. Uncertainties are expected in different steps of the
treatment planning, which should be statistically taken
into account in the planning process, resulting in an
optimized dose distribution or in some biologically
relevant figures. Accurate determination of irradiation
dose and volume to be treated is very important;
otherwise inaccuracy in these can cause severe
complications. This depends on the anatomic location,
histological type, tumour stage, potential lymph node
involvement, other tumour characteristics and normal
structures present in the area to be irradiated. In an
accurate delineation of target volume, BEV approach is
considered superior than the simulator approach. The
professionals involved in radiotherapy need to be aware
of developments in related fields. It is important that key
422
members of all the disciplines within radiotherapy be
aware of developments in different concerned areas.
1.
2.
3.
4.
5.
6.
7.
8.
9.
REFERENCES
Peterhans M, Frei D, Manser P, Aguirre MR, Fix MK. MonteCarlo dose calculation on deforming anatomy. Z Med Phys 2011;
21:113-23.
Buzdar SA, Gadhi MA, Rao MA, Laghari NA, Anees M.
Verification of computerized treatment planning for HDR 192Ir
Brachy therapy for gynaecological cancer. J Pak Med Assoc
2009; 59:113-5.
Hayashi N, Obata Y, Uchiyama Y, Mori Y, Hashizume C,
Kobayashi T. Assessment of spatial uncertainties in the
radiotherapy process with the Novalis system. Int J Radiat Oncol
Biol Phys 2009; 75:549-57.
Zizzari A, Michaelis B, Gademann G. Optimal feature functions
on co-occurrence matrix and applications in tumour recognition.
Greece: proceedings of the IASTED International Conference on
Applied Simulation and Modelling; 2002.
Zizzari A, Seiffert U, Michaelis B. Detection of tumour in digital
images of the brain. Greece: Proceedings of the IASTED International Conference on Applied Simulation and Modelling; 2001.
International Commission on Radiation Units and Measurements
(ICRU). ICRU report 23. Bethesda: ICRU; 1973.
International Commission on Radiation Units and Measurements
(ICRU). ICRU report 62. Bethesda: ICRU; 1999.
International Commission on Radiation Units and Measurements
(ICRU). ICRU report 50. Bethesda: ICRU; 1993.
Dobbs J, Barett A, Ash D, editors. Practical radiotherapy
planning. 3rd ed. London: Horald Arnold; 1999.
10. Seddon B, Bidmead M, Wilson J, Khoo V, Dearnaley D. Target
volume definition in conformal radiotherapy for prostate cancer:
quality assurance in the MRC RT-01 trial. Radiother Oncol 2000;
56:73-83.
11. Byhardt RW, Cox JD, Hornburg A, Liermann G. Weekly
localization films and detection of field placement errors. Int J
Radiat Oncol Biol Phys 1978; 4:881-7.
12. Hahn P, Shalev S, Viggaw D, Therrien P. Treatment planning for
protocol-based radiation therapy. Int J Radiat Oncol Biol Phys
1990; 18:937-9.
13. Urie MM, Goitein M, Doppke K, Kutcher JG, LoSasso T, Mohan
R, et al. The role of uncertainty analysis in treatment planning.
Int J Radiat Oncol Biol Phys 1991; 21:91-107.
14. International Commission on Radiation Units and Measurements. Dose specification for reporting external beam therapy
with photons and electrons. Draft Version; 1991.
15. Wu VW, Kwong DL, Sham JS. Target dose conformity in
3-dimensional conformal radiotherapy and intensity-modulated
radiotherapy. Radiother Oncol 2004; 71:201-6.
16. Buzdar SA. Optimization of accuracy concepts in treatment
planning for conformal radiotherapy [Thesis]. Bahawalpur: The
Islamia University of Bahawalpur, Pakistan; 2009.
17. Bentel C, editor. Treatment planning and dose calculation in
radiation oncology. 4th ed. New York: McGraw Hill; 1989.
18. Mechalakos J, Yorke Mageras GS, Hertanto A, Jackson A,
Obcemea C, Rosenzweig K, et al. Dosimetric effect of
respiratory motion in external beam radiotherapy of the lung.
Radiother Oncol 2004; 71:191-200.
19. Bengua G, Ishikawa M, Sutherland K, Horita K, Yamazaki R,
Journal of the College of Physicians and Surgeons Pakistan 2013, Vol. 23 (6): 418-423
Accuracy requirements in radiotherapy treatment planning
Fujita K. Evaluation of the effectiveness of the stereotactic body
frame in reducing respiratory intrafractional organ motion using
the real-time tumour-tracking radiotherapy system. Int J Radiat
Oncol Biol Phys 2010; 77:630-6.
20. LoSasso TJ. Three-dimensional conformal radiation therapy
Bristol. Philadelphia: Institute of Physics Publishing; 1993.
21. Moda N. Photon treatment planning collaborative working group;
“Three-dimensional dose calculations for radiation treatment
planning.” Int J Radiat Oncol Biol Phys 1991; 21:25-36.
22. Chen GT, Spelbring DR, Pelizzari CA, Balter JM, Myrianthopoulos LC, Vijayakumar S, et al. The use of beam's eye view
volumetric in the selection of non-coplanar radiation portals. Int
J Radiat Oncol Biol Phys 1992; 23:153-63.
23. Hamilton RJ, Kuchnir FT, Sweeney P, Rubin SJ, Dujovny M,
Pelizzari CA, et al. Comparison of static conformal field with
multiple non-coplanar arc techniques for stereotactic radiosurgery or stereotactic radiotherapy. Int J Radiat Oncol Biol Phys
1995; 33:1221-8.
24. Kutcher GJ, Burman C, Moban R. Compensation in threedimensional non-coplanar treatment planning. Int J Radiat Oncol
Biol Phys 1991; 20:127-34.
25. Van't Riet A, Mak CA, Moerland MA, Elders LH, Van der Zee W.
A conformation number to quantify the degree of conformity in
brachytherapy and external beam irradiation: application to the
prostate. Int J Radiat Oncol Biol Phys 1997; 37:731-6.
26. Wamberise A, Landberg T. ICRU report 62: Prescribing,
recording and reporting photon beam therapy. Supplement to
ICRU report 50; 1999.
27. Purdy JA, William BH, Matthews JW, Drzymala R, Emami B,
Simpson JR, et al. Room view display with real time interactivity.
Int J Radiat Oncol Biol Phys 1993; 27:933-44.
28. Buzdar SA, Rao MA, Nazir A. An analysis of depth dose
characteristics of photon in water. J Ayub Med Coll Abbottabad
2009; 21:41-5.
29. Buzdar SA, Afzal M, Todd-Pokropek A. Comparison of pencil
beam and collapsed cone algorithms, in radiotherapy treatment
planning for 6 and 10 MV photon. J Ayub Med Coll Abbottabad
2010; 22:152-4.
30. Amadeo. Conformal radiation therapy. New York: American
College of Radiology; 1998.
31. Michalski M. Physics of radiation oncology. St. Louis: Radiation
Oncology Centre, Washington University; 1996.
32. Kutcher GJ, Burman C. Calculation of complication probability
factors for non-uniform normal tissue irradiation: the effective
volume method. Int J Radiat Oncol Biol Phys 1989; 16:1623-30.
33. Lyman JT, Wolbarst AB. Optimization of radiation therapy, III: a
method of assessing complication probabilities from dosevolume histograms. Int J Radiat Oncol Biol Phys 1987; 13:103-9.
34. Niemierko A, Goitein M. Implementation of a model for
estimating tumour control probability for an inhomogeneously
irradiated tumour. Int J Radiat Oncol Biol Phys 1993; 29:140-7.
35. Perez CA, Purdy JA, Harms W, Gerber RL, Mathews JW,
Grigsby PW, et al. Design of a fully integrated three-dimensional
computed tomography simulator and preliminary results. Int J
Radiat Oncol Biol Phys 1994; 30:887-97.
36. Goitein M, Abrams M. Multi-dimensional treatment planning: II
Beam's eye view back projection, and projection through CT
sections. Int J Radiat Oncol Biol Phys 1993; 9:789-97.
37. Low N, Rosenberg S, Rubin I, Virudachalam S, Spelbring R, et al.
Beams eye view based prostate treatment planning: Is it useful?
Int J Radiat Oncol Biol Phys 1990; 19:759-68.
38. McShan DL, Fraass BA, Lichter AS. Full integration of the beams
eye view concept into computerized treatment planning. Int J
Radiat Oncol Biol Phys 1990; 18:1485-94.
39. Vijaykumar S, Low N, Chen GT, Myrianthopoulos L, Culbert H,
Chiru P, et al. Beams eye view based photon radiotherapy. Int J
Radiat Oncol Biol Phys 1991; 21:1575-86.
40. Perez CA. 3-D CRT or IMRT in localized carcinoma of the
prostate. Argentina: Association of Argentina Clinic; 2004.
41. Allan Hounsell, Transition from 2-D to 3-D conformal radiation
therapy- provision of a 3-D service. Manchester: north western
medical physics, Christie Hospital NHS Trust; 1994.
42. Nyholm T, Nyberg M, Karlsson MG, Karlsson M. Systematisation
of spatial uncertainties for comparison between a MR and a CTbased radiotherapy workflow for prostate treatments. Radiat
Oncol 2009; 4:54-62.
43. Pinkawa M, Pursch-Lee M, Asadpour B, Gagel B, Piroth MD,
Klotz J, et al. "Image-guided radiotherapy for prostate cancer.
Implementation of ultrasound-based prostate localization for the
analysis of inter- and intrafraction organ motion. Strahlenthe
Onkol 2008; 184:679-85.
44. Mageras GS, Podmaniczky KC, Mohan R. A model for computercontrolled delivery of 3-D conformal treatment. Med Phys 1992;
19:945-53.
45. De Boer HC, vanSornsen de Koste JR, Senan S, Visser AG,
Heijmen BJ. Analysis and reduction of 3-D systematic and
random setup errors during the simulation and treatment of lung
cancer patients with CT-based dose planning. Int J Radiat Oncol
Biol Phys 2001; 49:857-68.
46. Halperin R, Roa W, Field M, Hanson J, Murray B. Setup
reproducibility in radiation therapy for lung cancer: a comparison
between T-bar and expended foam immobilization devices. Int J
Radiat Oncol Biol Phys 1999; 43:211-6.
47. Samson MJ, van Sornsen de Koste JR, de Boer HC, Tankink H,
Verstraate M, Essers M, et al. An analysis of anatomic landmark
mobility and setup deviations in radiotherapy for lung cancer. Int
J Radiat Oncol Biol Phys 1999; 43:827-32.
48. Lax I, Blomgren H, Naslund I, Svanstrom R. Stereotactic radiotherapy
of malignancies in the abdomen. Acta Oncol 1994; 33:677-83.
49. Hunt MA, Kutcher JG, Burman C, Fass D, Harrison L, Leibel S,
et al. “The effect of setup uncertainties on the treatment of nasopharynx cancer”. Int J Radiat Oncol Biol Phys 1993; 27:437-47.
50. Bedford JL, Viviers L, Guzel Z, Child PJ, Webb S, Tait DM.
A quantitative treatment planning study evaluating the potential
of dose escalation in conformal radiotherapy of the oesophagus.
Radiother Oncol 2000; 57:183-93.
51. Kam MK, Chau RM, Suen J, Choi PH, Teo PM. Intensitymodulated radiotherapy in nasopharyngeal carcinoma:
dosimetric advantage over conventional plans and feasibility of
dose escalation. Int J Radiat Biol Phys 2003; 56:145-57.
52. Martinez AA, Yan D, Lockman D, Brabbins D, Kota K, Sharpe M,
et al. Improvement in dose escalation using the process of
adaptive radiotherapy combined with three-dimensional
conformal or intensity-modulated beams for prostate cancer.
Int J Radiat Oncol Biol Phys 2001; 50:1226-35.
53. Zhu SY, Mizowaki T, Norihisa Y, Takayama K, Nagata Y, Hiraoka
M. Comparisons of the impact of systematic uncertainties in
patient setup and prostate motion on doses to the target among
different plans for definitive external-beam radiotherapy for
prostate cancer. Int J Clin Oncol 2008; 13:54-61.
Journal of the College of Physicians and Surgeons Pakistan 2013, Vol. 23 (6): 418-423
423