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The Radiographer- vol. 49:141-146:
Validating the use of Digitally Reconstructed Radiographs
as Verification Tools in Radiation Therapy Simulation
of Prostate Treatment
J. Parker, 1 M. Vigar, 1 S. Dempsey, 1 H. Warren-Foiward, 1 & K. Jovanovich, 1•2
ABSTRACT
Purpose: The aim of this study was to validate the clinical usefulness of Digitally Reconstructed Radiographs (DRRs) as a
replacement tool for simulator verification films in prostate treatment.
Method: The study was performed on a convenience sample of 13 patients who had all undergone or were currently undergoing prone treatment to the prostate and/or seminal vesicles. All patients received 60 or 66Gy in 30 or 33 fractions via a fourfield PA/AP, Lt. & Rt. lateral field technique using 6 or 18 MV photons. The previously acquired PA and lateral simulation
films were collected for each patient and a set of DRRs produced from stored
data. In total 26 DRRs were produced and
these were compared to the 26 simulation films. In order_ to compare and identify variation between the image sets distance
measurements were taken from a single bony landmark to the isocentre in the superior-inferior (sup-inf), left-right (lt-rt), and
anterior/posterior (ant-post) directions.
Results: In the sup-inf displacement a maximum difference of 0.4cm was ol)served (correlation coefficient of r = 0.96, p<0.001).
In the lt-rt displacement a maximum difference of 0.3cm was observed (rr'= 0.89, p<0.001). In the ant/post direction an outlying difference of 0.8cm was recorded although 10 of 13 measurements were less than 0.3cm (r=0.94, p<O.OOl).
Conclusion: A statistically significant correlation was found between the image sets. In terms of isocentre placement, DRRs
within this study accurately represented the simulation verification film and may be considered as a clinically acceptable
replacement for the current simulation verification film in prostate treatment.
This outcome suggests that current clinical protocols could change to incorporate the increased use of DRRs as either aids
in placing planned fi¢ld position within a verification simulator session or as a primary planning output replacing the current
simulation verification session. This has implication for both the patient and the department.
GT
INTRODUCTION
Images from modalities such as computed tomography (CT),
magnetic resonance imaging (MRI) and positron emission
tomography (PET), downloaded onto a radiation therapy planning system, can provide a 3D model of the patient's anatomy
that allows more accurate identification of planning target volumes (PTV) and their relationship with organs at risk (OAR).
Modern radiation therapy planning systems also facilitate the
use of these images to create divergently correct digitally
reconstructed radiographs, which can look similar to simulation images, and which can display volumes, field shape and
isodose distributions.'
Purdy (1997, p358)2 at the time defined digitally reconstructed
radiographs (DRRs) as " ... 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 3D planning." Once
I. Faculty of Medicine & Health Sciences, The University of Newcastle,
Australia
2. Radiation Oncology, Mater Hospital, Newcastle, Australia
Address for Correspondence:
Mr Shane Dempsey,
Box 16,
'
Hunter Building, University of Newcastle
NSW Australia
Tel: +61 2 4921 6667
Fax: +61 2 4921 7053
E-mail: [email protected]
econstructed, the DRR can serve as a reference image for transferring the 3D-treatment plan to the treatment machine. The
DRR can replace the conventional simulator radiograph with a
digital version obtained by arrangement of the CT voxels into
a geometrically equi'\_l!lent planar view. It should be noted that
the data set can include imaging modalities other than CT.,_.
For clinical usefulness DRR images must give reasonable
approximation- of true simulation images in terms of both spatial resolution and subject contrast so that multiple bony landmarks can be used to compare the DRR to a simulator or treatment verification image. 7
The increa&ing potential for clinical use of DRRs as a referenc'e image for treatment field and set-up verification can be
explained through understanding the inherent advantages these
images possess over current hard copy plain film images.
ADVANTAGE 1: Because DRRs are directly derived from CT
data their geometric accuracy, with respect the treatment plan,
is not compromised by inaccuracies in the treatment simulation. An example of this is the reduction of manual judgement,
or parallax error, in visually adjusting the treatment fields, via
manual fluoroscopic movement as performed currently from
the plan in the simulator verification session.
ADVANTAGE 2: The DRR computation can be manipulated
to change the appearance of the image to facilitate comparison
with portal images. An application of this is where the DRR can
be digitally manipulated to improve contrast or resolve anatomy by selectively removing overlying anatomical structures.
ADVANTAGE 3: DRRs can be computed for arbitrary angles
of projection through CT slices. This allows for quantification
of patient rotation along axes parallel to the plane of the imag-
The Radiographer- vo/.49, no. 3, December 2002
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VALIDATING THE USE OF DIGITALLY RECONSTRUCTED RADIOGRAPHS
~.
AS VERIFICATION TOOLS IN RADIATION THERAPY SIMULATION OF PROSTATE TREATMENT
ing device, and out of plane rotations can then be verified.
ADVANTAGE 4: With constant advancements in computing
technologies, such as improvements in hardware processing
and therefore faster computational ray tracing algorithms,
DRRs are now produced within, clinically acceptable times.
This study attempts to validate the clinical usefulness of
DRRs as a replacement tool for simulator verification films.
Prior to implementing into clinical practice emerging technologies, like DRRs, there is a need to validate the new technology
or protocol against existing protocols to assess the outcomes,
benefits and disadvantages of the technology or protocol.
Validation studies are an important element of acceptance testing. Validation studies also allow the ethical considerations of
the technology or protocol, ie. the real or perceived risks and
benefits, to be considered in isolation of the rush to implement
the new procedure into clinical practice.
In this study comparisons were made between DRRs produced on the planning system and the films currently taken at
the simulator verification session by compari!,lg the var.iation
between the image sets. Variation was assessed as the distance
measured from a single bony landmark to the intended treatment isocentre in the sup-inf, lt-rt, and ant-post directions.' If
there is significant correlation between the image sets this may
suggest that:
1. DRRs can be produced on the current planning system with
comparable image quality to simulatpr radiographs making
·
_,
comparison possible.
2. DRRs are valid and accurate tools for verifying isocentre
location with respect to pelvic bony anatomy and may
replace simulation verification images with respect to
establishing the treatment isocentre location.
3. DRRs can be used as the reference image for treatment verification!;10
\
METHOD AND MATERIALS
A convenience sample of 20 patients who were being treated,
or had completed a radical course of radiation therapy to the
prostate, or prostate and seminal vesicles, at the Mater Hospital
Newcastle, were selected for this validation study on thf1 use of
DRRs as a verification tool within a radiotherapy depj~rtment.
To be considered eligible for the study all patients haq to positioned prone and have undergone a traditional4 field- PNAP &
Rt/Lt lateral "box" technique. All patients received between
60Gy to 66Gy to the isocentre, (ICRU50 reference point) in 30
or 33 fractions, using 6 or 18 MV photons.
;.
All patients had planning CT's performed u~ing a Seimens
Somatron Plus 4 CT scanner. Each CT study consisted of 5mm
thick contiguous slices through the intended treatment area
from which volumes and plans where produced. All patients
were planned using the ADAC
Pinnicle' 3D planning system
Version 5.0e, and all plans
Right: Figure 1: ·
were produced prior to the · Three Dimensional Co-ordinate
verific1,1tion simulation sesSystem. The x-axis represents
sions. These plans, along with
left-right displacements as
the CT scout image, were used
determined from the PA film.
by radiation therapists to visuThe y-axis represents superiorally place the treatment fields
inferior displacements as
during simulation verification
determined from the PA film.
and these simulator verification
The z-axis represents anteriorfilms
were
used
for
posterior displacements as
subsequent comparison with
determined from the lateral
treatment verification images
film.
142
The Radiographer- vol. 49, no. 3, December 2002
(port films).
In assessing DRRs within this study prostate treatment was
chosen for the following reasons:
The prostate is generally a mid-sagittal plane structure, and
isocentre localisation during simulation may vary from the
planned
isocentre
position in sup-inf, lt-rt, and ant-post directions. This scope
of possible movement in three dimensions will allow adequate quantification of the differences between the image
sets.
The surrounding bony anatomy in relation to the prostate is
stable, with multiple bony landmarks from where measurements can be taken to compare the accuracy of the planned
isocentre on the two images.
Upon analysing the cases currently stored on the hospitals
planning system, there were a sufficient number of prostate
treatments to collect a pilot or validation study sample, and
those selected should reasonably represent patients typically being referred for treatment to this centre.
In assessing the completed plans for the most recent twenty
prostate patients treated, seven patients were removed from the
study. Three patients were ruled out because they were positioned supine, therefore comparisons of variability of anatomy
position could not be made. Data from three patients were
unable to be retrieved from the computer for technical reasons,
and the data for one patient was incomplete at the time of the
research. In all 13 patients were included in this study and 26
DRRs were produced, comprising one P;A and one lateral DRR
per patient.
DRRs were produced using the following parameters. For
the PA DRR an energy of 0.0006 MeV and a brightness level
of 0.6 to 0.8 was selected, and for the lateral DRR an energy of
0.0002 MV and brightness level of 0.5 to 0.8 was selected.
These are average exposures and were yaried slightly depending on the size of the patient. (Note: Optimal DRRs are now
produced using the default values on Pinnacle 6.2G). For visual comparison to the simulation films the DRRs were printed
on plain paper using a Hewlett Packard Laser printer. The
crosshairs and field outlines were left on for the purposes of the
research.
The anatomical reference points used for comparison
y
----------~~~~~--------X
z
J PARKER, M. VIGAR, S. DEMPSEY, H. WARREN-FORWARD, K. JOVANOVICH
tion therapy a value of (0.5cms has historically been used
as the point at which image sets are thought to agree. With
the shift to more conformal beam shapes and better patient
immobilisation methods this value could realistically be
reduced to s0.3cms or less.
between the image sets were the right anterior aspect of symphysis pubis (RAASP) for the anterior films and the anterior
aspect of the symphysis pubis (AASP) for the lateral views.
Once the DRRs were produced the simulation verification
films were located a~d distances from the isocentre to the
anatomical landmarks on both image types were recorded on a
worksheet and the difference between the image sets
determined.
The measurements on both images sets were considered in
three dimensions as displayed in the Figure 1.
This research was carried out over approximately a five
month period as part of a supervised final year student research
project. The researchers reviewed the planning records to identify a large enough pool of similar cases that could be used for
the study, learnt to generate and optimise DRRs on the Pinnacle
planning~ system, identified bony landmarks suitable for comparison, and learnt to use and interpret a statistical data base.
The actual production of DRRs for all cases and measurement
of differences between all image sets took 2-3 weeks and was
dependent on the access to the planning system. It would be
possible to perform the DRR production and measurements in
a much shorter time frame if they formed part of the routine
planning protocol. Basic data analysis was performed using
Microsoft Excel.
Assessment of how well the DRRs correlated with the simulation images was made on two levels.
I. Statistical analysis: Scattergrams of the measured differences between images sets were generated and correlation
coefficients and the associated p-values were determined
for agreement in 1,1ll three directions separately. A correlation coefficient greater than 0.8 indicates a high level of
correlation between the image sets.
2. Descriptive analysis: Bar Charts were generated that
showed the actual agreement and differences between
image sets. When comparing verification images in radia-
RESULTS
The comparison of the measured differences between the
images sets in the sup-inf, lt-rt, and ant-post directions, demonstrated a statistically significant correlation between DRRs and
simulation verification films for the purposes of isocentre localisation for prostate treatments. Given that the results displayed
a similarly high level of correlation between the image sets in
all three directions, individual results have been displayed only
for the assessment of one set of images, the left to right displacements (Figures 2 & 3). Figure 4 displays the actual differences between images sets when all data is pooled or combined. Table 1 provides an overview of the levels of agreement
and statisticaVpnalysis of the entire data set.
Figure 2 cl1splays the differences between the DRRs and the
simulator verification films in terms of lateral isocentre displacement from the anatomical bony landmark, measured from
the AP/PAview. For five image sets (5 out of 13) the isocentre
displac'iment between the DRR and the simulator verification
film d~inonstrated 100% agreement. Twelve image sets (12 out
of 13) had an overall difference (0.2cm with the maximum difference recorded being 0.3 em for one patient. These r.esults fall
within the historic 0.5cm tolerance, which is generally accepted at the centre as being within clinical tolerances.
Figure 3 is a scattergram plotting the left to right isocentre
displacements for both imaging modalities. Correlation analysis demonstrates a high level of statistically significant agreement between the DRRs and the simulator radiograph (r = 0.89,
p = <0.001).
Figure 4 is a scattergram plotting the pooled isocentre
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The Radiographer- val. 49, no. 3, December 2002
143
VALIDATING THE USE OF DIGITALLY RECONSTRUCTED RADIOGRAPHS
~.
AS VERIFICATION TOOLS IN RADIATION THERAPY SIMULATION OF PROSTATE TREATMENT
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thin dotted line represents the y=x line between both image sets and the thick line represents the line
of best fit for the measurements obtained.
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\
displacements in sup/inf, lt/rt, and ant/post directions for both
imaging modalities. Correlation analysis demonstrates a high
level of statistically significant agreement between the DRRs
and the simulator radiograph (r = 0.99, p = <0.001).
Table 1 provides an overview of the level of agreement and
144
The Radiographer- vol. 49, no. 3, December 2002
the differences between individual and pooled image sets, as
well as the correlation co-efficients for. the data sets. Perhaps
the most important observation to be mad.e from the table is that
for the pooled data there were 34 of 39 measured displacements
between the image sets that were s0.3cm.·
J.PARKER, M. VIGAR, S. DEMPSEY, H. WARREN-FORWARD, K. JOVANOVICH
Table 1: Descriptive and statistical comparison of all individual
image sets and the p9oled data.
Left to Right Lateral Displacements from AP/PA image sets
100% isocentre agreement
5 out of 13 image sets
Differences s0.3cm
12 out of 13 image sets
Maximum difference
0.3cm
Correlation Coefficient, p-value
r = 0.89, p=<0.001
Superior/Inferior Displacements from AP/PA image sets
100% isocentre agreement
2 out of 13 image sets
Differences s0.3cm
11 out of 13 image sets
Maximum difference
0.4cm
Correlati6n-coefficient, p-value
r = 0.96, p=<0.001
Anterior/Posterior Displacements from lateral image sets
100% isocentre agreement
0 out of 13 image sets
Differences s0.3cm
11 out of 13 image sets
Maximum difference
0.8cm
Correlation Coefficient, p-value
r = 0.94, p = <0.001
Pooled Displacements from all images
100% isocentre agreement
7 out of 39 image sets
Differences s0.3cm
34 out of 39 image sets
Maximum difference
0.8cm
Correlation Coefficien!, p-value
r = 0.99, P=<0.001
DISCUSSION
This study demonstrated, within the framework of how prostate
planning and simulation is performed at the Mater Hospital,
Newcastle, that DRRs \ire valid and accurate tools for verifying
isocentre location with respect to pelvic bony anatomy, and
therefore may replace simulation verification images with
respect to establishing the treatment isocentre location in
prostate treatments.
With this result comes the idea that for those treatment
regions where validation of the DRR to the simulator film has
been demonstrated, the DRR can be used to establish the treatment isocentre position from the developed plan without the
need for a post planning simulation session. This validation
also means that the DRR could serve as the reference image for
comparison with subsequent treatment verification images, but
this suggested outcome would need to be validated in a study
comparing treatment verification images to simulation reference images and the DRR reference image.
Within this study it might have been possible to achieve a
higher level of correlation if the quality of DRRs produced
were improved. In this study the CT slice thickness used was
5mm, and this did not provide good bone edge definition, making it difficult to accurately measure isocentre displacements
from bony landmarks. This problem could have been the reason
why we obtained many differences that were between 0.1 and
0.3cm. By simply reducing the slice thickness to 3mm, the
quality of the DRRs should improve allowing more accurate
measurements to be obtained however there may be patient
dose implications that need to be considered before doing this.
This research uncovered a need for independent cross
checking by another person, or the multiple measuring of data
by the research group, to reduce measurement errors and
increase measurement validity. It was found for one patient that
the initial measurements taken from the isocentre for one DRR
image were not de-magnified and therefore a large difference
was noted between this and the simulation image. When this
was discovered all measurements within the study were rechecked and the results changed for that patient increasing the
resulting couelation analysis.
Having introduced the discussion point of human error in
measuring the DRRs and simulator radiographs, it should also
be noted that the correlation might have been influenced by the
error involved with the simulating therapist visually placing the
fields in the simulation verification session. This is a source of
potential error that needs to be ascertained.
Care needs to be taken when generalising the results of this
study to other departments or sites of treatment other than the
prostate. There may be differences between clinical centres as
to how patient~ are planned, simulated and treated. These differences may' be enough to require clinical sites to undertake
their own replication studies to ensure internal validity of the
outcomes.
It wcpf mentioned in the introduction that there are ethical
considerations to implementing new technology or new protocols. It;is not necessarily ethical to only think that a new technology may offer benefits and therefore should be rushed into
clinical service. It is our responsibility to ensure through
research that this is in fact the case, and that there is no possibility for the implementation of the technology or protocol to in
fact do harm. This study validated the technology and protocol
development of the use of DRRs priot to actual clinical use,
and demonstrated equivalence with the current clinical methods used within the department. Benefits to the patient would
include a reduction in the need for a simulation verification session leading to a reduction of patient time spent in preparation
for treatment and decreasing patient radiation dose from simulation exposures. Possible harm could arise if the plan or DRR
is developed incorrectly and . therefore treatment isocentre
" localisation was inco~ct. This issue could be checked by visual inspection of the DRR against the marked CTs or by pretreatment verification images, ie port film or EPI.
CONCLUSION
This validation study has concluded on the basis of the highly
statistically significant correlation analysis, that the DRRs produced as part of this prostate study do visually and quantitatively represent the isocentre position. The image sets demonstrated agreement of the isocentre position in x, y and z directions when compared to the current verification techniques
employed at the clinical centre used within the study. The
implication for the clinical centre may be a refinement of their
current protocol to include the use of DRRs as both the initial
source of isocentre position and also as the reference image for
treatment verification. This notion has positive efficiency gains;
for the department in terms of reducing simulation verification
sessions; and for the patient in removing the need for an additional simulation session and resulting radiation dose.
The results of this should now be further validated by a
larger prospective randomised study which would take into
account differences between patient physical characteristics,
and monitored for the entire length of the patient's treatment
using the DRR as the source of portal image verification. Also
DRRs should be validated in each individual centre to allow for
The Radiographer- val. 49, no. 3, December 2002
145
VALIDATING THE USE OF DIGITALLY RECONSTRUCTED RADIOGRAPHS
~
AS VERIFICATION TOOLS IN RADIATION THERAPY SIMULATION OF PROSTATE TFfEATMENT
the differences with each centre to be included in the evaluation
ofDRRs.
Another conclusion drawn from the study is that a reduction in CT slice thickness would increase the quality of the
DRRs produced and hence improve the quality of the images
for use as a verification tool for prostate treatments which
should make the measurement of differences more precise.
The method we employed in this study is quite easy to perform and is reproducible for other treatment sites. The method
could be replicated as part of a constant quality improvement
procedure for other anatomical sites.
Author's note
Due to the outcomes of this study a larger randomised prospective study, as mentioned in the conclusion, is to be undertaken on prostate patients at the ~ater Hospital, NSW,
Australia.
With thanks to
Mr Steve Howlett, Senior Medical Physicist, The Mater
Hospital, NSW, Australia.
The staff and patients of the Newcastle Mater Misercordae
Hospital for their time and co-operation
The work expressed in this paper owes much to the intellectual and supportive environment of The Discipline of
Medical Radiation Science, Fac'~lty of Health, The
University of Newcastle, Australia.
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Peer reviewed
Submitted: June 2002
Accepted: October 2002