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www.ijcto.org

Corresponding author: Shanmugam Thirumalai Swamy; Department of Radiation Oncology, Yashoda Hospitals, Hyderabad, India.
Cite this article as: Thirumalai-Swamy S, Anuradha C, Kathirvel M, Arun G, Subramanian S. Pretreatment quality assurance of volumetric modulated arc
therapy on patient CT scan using indirect 3D dosimetry system. Int J Cancer Ther Oncol 2014; 2(4):020416. DOI:10.14319/ijcto.0204.16

Pretreatment quality assurance of volumetric modulated arc
therapy on patient CT scan using indirect 3D dosimetry system

Shanmugam Thirumalai Swamy1,2, Chandrasekaran Anuradha2, Murugesan Kathirvel1,
Gandhi Arun1, Shanmuga Subramanian1

1Department of Radiation Oncology, Yashoda Hospitals, Hyderabad, India
2School of Advanced Sciences, VIT University, Vellore, India

Received August 29, 2014; Revised October 16, 2014; Accepted October 18, 2014; Published Online October 22, 2014

Original Article

Abstract
Purpose: Aim of this study is to clinically implement the COMPASS 3D dosimetry system for pretreatment quality assurance of
volumetric modulated arc therapy (VMAT-RapidArc) treatment plans. Methods: For this study, 10 head and neck (H&N) and
pelvis VMAT plans dose response from Linac was measured using COMPASS system along with MatriXXEvolution and 3D dose
reconstructed in the patient computed tomography (CT) scan. Dose volume histograms and 3D gamma were used to evaluate
difference between the measured and calculated values. In order to validate the COMPASS system, dose response for open fields
were acquired for both homogeneous and inhomogeneous phantoms. Results: The average dose difference between Eclipse
treatment planning system (TPS) calculated and COMPASS measured (homogenous medium) in normalization region, inner
region, penumbra region and buildup region was less than ±2%. In inhomogeneous phantom, there was a maximum difference
-3.17% in lung, whereas the difference other densities was within ±2%. The systematic increase in the average 3D gamma be-
tween the TPS calculated and COMPASS measured for VMAT plans with known dose errors and multi-leaf collimator (MLC)
offset errors shows that COMPASS system was sensitive enough to find clinical significant errors. The 3D dose parameters (D95,
D1, and average dose) of all H&N and pelvis patients were well within the clinically acceptable tolerance level of ±5%. The
average 3D gammas for planning target volumes (PTV) and organ at risks (OAR) of the patients were less than 0.6. Conclusion:
The results from this study show that COMPASS along with MatriXXEvolution can be effectively used for pretreatment verification
of VMAT plans in the patient anatomy.

Keywords: COMPASS; VMAT; 3D Gamma; Pretreatment QA

Introduction
In advanced radiotherapy techniques, each patient's treat-
ment plan is customized, to produce high gradient dose dis-
tribution to the target and low dose to the critical organs.
Volumetric Modulated Arc Therapy (VMAT- RapidArc)
produces highly conformal dose distribution by simultane-
ously changing multi-leaf collimator (MLC) position, dose
rate and gantry speed during patient treatment.1-3 Complex
treatment deliveries demand a comprehensive quality assur-
ance (QA) procedure. American Association of Physicists in
Medicine Task Group (AAPM TG)-824 recommends verifica-
tion of intensity modulated treatment plans with an inde-
pendent QA method before treatment delivery. Traditionally
pretreatment QA’s are performed in a phantom using ion
chamber, film, 2D array, and electronic portal imaging de-
vice (EPID). Each of these devices has been proven useful
but has its own limitations.5-11 Position of ion chamber in the
high dose gradient area leads to discrepancy between meas-

ured and the treatment planning system (TPS) calculated
dose. Film shows excellent spatial resolution; however,
problems like chemical processing, scanner readout, and
time delay exists. A 2D array shows the dose distribution
immediately after the treatment delivery, but their results
have limited resolution. Also, results provided by them can-
not be directly used to identify delivery errors in tumor and
normal tissues. Benjamin et al.12 showed that there is lack of
correlation between gamma passing rates from 2D array and
dose differences in critical anatomic regions of interest. To
address this issue, alternate QA techniques has been devel-
oped to verify the 3D dose distribution by measuring fluence
at different gantry angle using ion chamber matrix or EPID
in a patient computed tomography (CT) scan.13-18 The
COMPASS QA system (IBA Dosimetry, Germany) is one
such technique which uses MatriXXEvolution along with gantry

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angle sensor. It consists of (i) a measurement-based dose
reconstruction, and (ii) model-based dose calculation.

Measurement based dose reconstruction
(i) Detector dose response is predicted from the patient
treatment plan parameters. (ii) Measured dose response from
the MatrixEvolution is then compared with predicted dose re-
sponse. (iii) The difference between the predicted and meas-
ured response along with correction kernel was used to de-
rive the reconstruction fluence. Finally reconstructed flu-
ence is fed to the dose engine based on collapsed cone model
for computation of 3D dose within the patient CT scan. The
dose calculated from the reconstructed fluence is referred as
“indirectly measured” (COMPASS measured).

Model based dose calculation
COMPASS system can compute dose in patient CT scan us-
ing inbuilt beam model. The purpose of the dose computa-
tion is to provide an independent cross-verification of TPS
calculated dose.

In addition, COMPASS has a facility to compare the 3D dose
distribution and dose volume histograms (DVH) between
measured and TPS calculated. Previous studies have validat-
ed the dose reconstruction method of COMPASS system for
fixed field IMRT against film based QA in phantoms.13, 15, 17
Ramesh et al.14 has shown the experimental validation of
COMPASS system for rotational IMRT in inhomogeneous
phantom. This study focuses on pretreatment QA of VMAT
treatment plan on “measurement based dose reconstruction”.
We have evaluated the performance of COMPASS 3D do-
simetry system clinically: (i) In open field sizes (10 × 10 cm2
and 20 × 20 cm2) to evaluate the accuracy of beam modeling
both in homogeneous and inhomogeneous media, (ii) To
check the accuracy of the system in detecting MLC and dose
errors and (iii) To implement the system for VMAT patient
specific QA.

Methods and Materials
All measurements were performed using 6 MV photon beam
from dual energy Clinac-iX (Varian Medical Systems, Palo
Alto, USA). The machine was equipped with millennium 120
MLC, on-board imager (OBI) and maximum dose rate of 600
MU/min. Conventional pretreatment QA for VMAT plans
were performed with point dose measurements and 2D pla-
nar dosimetry using 0.13 cc ionization chamber in a cube
phantom and MatriXXEvolution in multicube phantom (IBA
Dosimetry) respectively. MatriXXEvolution contains 1020 paral-
lel plane ion chambers (32 × 32 matrix) with an active area of
24.4 × 24.4 cm2 having 7.62 mm resolution at isocenter of 100
cm. In point dose measurement, percentage dose variation
was calculated between the measured dose using ion cham-
ber and the calculated dose in Eclipse TPS. For 2D planar
dosimetry (frontal plane), global gamma analysis was per-

formed using OmniPro Im’RT software with criteria of 3%
dose difference (DD) and 3mm distance to agreement (DTA).

In this study, COMPASS (V2.1) was used as a pretreatment
3D QA tool to validate VMAT plans. Dose response from
treatment plans was measured using MatriXXEvolution with 5
cm RW3 buildup plates and gantry angle sensor placed on a
gantry holder mount, Figure 1 (source to detector distance of
76.2 cm). The dose calculation engine in the COMPASS sys-
tem is collapsed cone convolution/superposition (CCC/S)
algorithm, whereas Eclipse TPS uses Analytical Anisotropic
Algorithm (AAA). Since these two algorithms use different
approach in dose calculation, simple open field sizes were
first validated in both homogeneous and inhomogeneous
mediums.

FIG.1: Setup for the pretreatment quality assurance for VMAT
treatment delivery. MatriXXEvolution with 5 cm RW3 buildup plates
and gantry angle sensor placed on a gantry holder mount with
source to detector distance-76.2 cm.

Open field measurements
To verify the accuracy of beam modeling and algorithm dif-
ference, the fluence for MLC defined open fields (ranging
from 5 × 5 cm2 to 25 × 25 cm2) were acquired by COMPASS
system on a homogeneous phantom. Average dose calculated
and measured by Eclipse TPS and COMPASS QA system
respectively were compared at normalization, inner, outer,
penumbra and buildup regions as per AAPM TG-53 19 rec-
ommendations (Figure 2a).

Average dose had been compared for different Hounsfield
unit (HU) mediums teflon (990 HU), derlin (340 HU), acrylic
(120 HU), polystyrene (-35 HU), LDPE (-100 HU), PMP
(-200 HU) and air (-1000 HU) in CAT Phantom (Figure 2b).
This average dose comparison was done for two sets of MLC
defined open fields (10 × 10 cm2 and 20 × 20 cm2) in an in-
homogeneous phantom.



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FIG.2 (a): Homogeneous phantom created in Eclipse TPS as per AAPM TG-53 recommendations. Based on beam geometry, phantom was bro-
ken into different regions to analyze the agreement between TPS calculated and COMPASS measured dose. FIG. 2(b): Transverse view of CAT

phantom CT scan with different HU medium.

FIG. 3: Dose distribution and DVH comparison between Eclipse TPS calculated and COMPASS measured for an H&N patient.

Intentional error measurements
To validate the accuracy of the COMPASS system, VMAT
plans with known dose error of ±3 and ±5% and MLC errors
of 1, 3 and 5 mm due to one side of MLC bank
mis-calibration were delivered. For 2D planar dosimetry,
dose plane from TPS plan without error was used as refer-
ence in OmniPro Im’RT software for gamma analysis. For 3D
dosimetry, VMAT plan without error was exported to
COMPASS system.

Patient plan evaluation
10 head & neck (H&N) and 10 pelvis VMAT plans with two
full arcs (181°-179° clock wise / counter clock wise) were
chosen for this study. The H&N patients contain three plan-
ning target volumes (PTV) with different level of dose pre-
scription (simultaneous integrated boost, Figure 3) and single
PTV for pelvis patients. VMAT plans were optimized in

Eclipse TPS (version 8.9) using Progressive Resolution Opti-
mizer -II and final dose calculations were performed using
AAA with 2.5 mm grid resolution. For pretreatment 3D QA,
these VMAT plans along with patient’s CT scan, structure set
and 3D dose planes were exported to COMPASS in DICOM
RT format.

On treatment machine, MatriXXEvolution (Trigger Mode) was
used for measuring dose response. COMPASS system calcu-
lates predicted dose response using DICOM RT plan param-
eters (gantry angle, MLC position, MU, etc) from TPS, de-
tector model and in-built beam model. This predicted dose
response was compared against the corresponding measured
dose response and the difference was incorporated in dose
calculation (Figure 4). The final dose distribution was recon-
structed on patient CT using CCC/S algorithm with same
grid size of 2.5 mm. The average doses for PTV’s and organs



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© Thirumalai Swamy et al. ISSN 2330-4049

at risk (OAR’s) in H&N as well as pelvis patients were com-
pared between TPS calculated and COMPASS measured.
Dose to 95% of the PTV volume (D95) and max dose to spi-
nal cord (D1) was also evaluated. The average 3D global
gamma was calculated using criteria of 3mm DTA and 3%
DD.

Results
Open fields
The percentage difference of average dose between TPS cal-
culated and COMPASS measured for different region and
HU material were shown in Figure 5a and 5b respectively. In
order to understand the dose difference arising from the
beam modeling, the open field measurements were analyzed.
Percentage difference of average dose for all open fields in
normalization, inner, penumbra and buildup regions was less
than AAPM TG-53 19 recommended value 0.5%, 1.5%, 10%
(2mm) and 20% respectively. For outer region suggested
value was 2%, but in our study due to low dose a maximum
difference of 8.3% was observed.

FIG. 4: The predicted dose response by COMPASS system was com-
pared against the corresponding measured dose response from Ma-
triXXEvolution. The difference in response was incorporated in final
dose reconstruction.

FIG. 5 (a): Percentage difference in average dose for different regions of interest in homogeneous phantom for MLC defined open fields size of 5
× 5 cm2, 10 × 10 cm2, 15 × 15 cm2, 20 × 20 cm2 and 25 × 25 cm2 respectively. FIG. 5 (b): Percentage difference in average dose for different HU

material in CAT phantom for MLC defined open fields 10 × 10 cm2 and 20 × 20 cm2

Intentional errors

Table 1 shows the gamma agreement index (GAI) for known dose and MLC errors between TPS calculated and MatriXXEvolution measured in
multicube phantom. GAI is defined as percentage of points passing the gamma evaluation criteria. COMPASS system provides the clinical rele-
vance of dose and MLC errors by calculating the average dose difference and 3D gamma for PTV, rectum and bladder.

TABLE 1: Shows gamma agreement index (GAI) for measurements with intentional dose and MLC errors, whereas COMPASS system provides
the clinical relevance of errors by calculating the average 3D gamma and dose difference for PTV, rectum and bladder.

Description GAI
average 3D gamma %  difference in average dose

PTV Rectum Bladder PTV Rectum Bladder
Without Error 97.5 0.33 0.28 0.25 -0.73 3.11 -2.77
3% Dose Error 94.8 1.03 0.27 0.55 -3.80 0.56 -5.28
5% Dose Error 89.9 1.50 0.38 0.69 -5.64 -2.66 -6.96
-3% Dose Error 92.4 0.58 0.40 0.21 2.04 5.22 0.58
-5% Dose Error 87.3 1.22 0.61 0.45 4.39 7.62 2.81
1mm MLC Error 94.3 0.45 0.28 0.5 -1.06 2.61 -6.53
3mm MLC Error 92.95 0.77 0.45 0.41 -1.61 2.18 3.33
5mm MLC Error 85.65 1.08 0.61 0.78 -0.96 1.30 7.93



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TABLE 2: 3D gamma and dose volume difference between Eclipse TPS calculated and COMPASS measured for 10 H&N patients PTV’s and
OAR'S. For same patients, point dose difference from ion chamber and 2D gamma from planar dosimetry were listed.

Parameters Structure Pt-1 Pt-2 Pt-3 Pt-4 Pt-5 Pt-6 Pt-7 Pt-8 Pt-9 Pt-10 Mean ±SD
% of point dose difference 0.97 1.07 -1.11 -0.67 -0.93 -1.29 -2.60 2.09 -2.45 0.63 -0.43 1.56
gamma agreement index 96.1 97.0 97.9 95.1 96.7 95.5 97.9 98.3 98.1 95.1 96.7 1.26

%  difference in average
dose

PTV1 -2.02 -1.53 -2.07 -2.46 -2.09 -1.94 -1.18 -1.25 -0.79 -2.97 -1.83 0.65
PTV2 -2.36 -1.47 -1.98 -2.69 -2.08 -1.36 -1.41 -1.38 -0.96 -2.42 -1.81 0.57
PTV3 -2.56 -0.35 -1.62 -1.88 -2.91 -1.37 -2.62 -2.65 -1.65 -2.71 -2.03 0.81

Rt Parotid 0.71 -4.00 -3.93 -1.89 4.74 -3.10 -2.75 2.38 -2.05 -4.52 -1.44 3.07
Lt Parotid -2.42 -2.69 0.33 3.76 3.13 -2.50 1.23 -0.51 -1.28 3.99 0.30 2.62
Spinal cord -2.95 -1.12 -2.87 -3.17 -0.83 -1.49 0.23 -1.23 -0.57 -1.18 -1.52 1.13

average 3D gamma

PTV1 0.37 0.33 0.49 0.31 0.41 0.57 0.40 0.36 0.27 0.44 0.40 0.09
PTV2 0.39 0.30 0.45 0.36 0.44 0.41 0.36 0.33 0.27 0.48 0.38 0.07
PTV3 0.43 0.24 0.30 0.41 0.46 0.52 0.46 0.56 0.40 0.54 0.43 0.10

Rt Parotid 0.14 0.30 0.08 0.22 0.25 0.40 0.22 0.11 0.13 0.30 0.22 0.10
Lt Parotid 0.31 0.26 0.18 0.22 0.29 0.28 0.14 0.09 0.11 0.14 0.20 0.08
Spinal cord 0.39 0.11 0.36 0.21 0.20 0.37 0.24 0.15 0.11 0.18 0.23 0.11

%  difference in D95
PTV1 -1.95 -2.83 -1.80 -2.22 -2.28 -1.05 -0.26 -0.91 -0.90 -2.50 -1.67 0.84
PTV2 -2.29 -1.10 -1.72 -2.23 -2.36 0.29 -0.99 -1.80 -0.52 -2.24 -1.50 0.89
PTV3 -2.20 0.86 -0.23 -0.84 -2.56 1.85 -1.35 -2.68 -0.16 -2.33 -0.96 1.54

%  difference in D1 Spinal cord -1.88 -1.00 -3.17 -1.14 -1.37 -2.84 -0.27 -3.60 0.85 -1.96 -1.64 1.36

TABLE 3: 3D gamma and dose volume difference between Eclipse TPS calculated and COMPASS measured for 10 pelvis patients.
Parameters Structure Pt-1 Pt-2 Pt-3 Pt-4 Pt-5 Pt-6 Pt-7 Pt-8 Pt-9 Pt-10 Mean ± SD

% of point dose difference 0.16 2.71 0.32 1.6 -0.91 0.1 0.93 0.16 -1.07 -1.11 0.29 1.22
gamma agreement index 98.4 97.2 98.6 99.1 95.1 96.4 97.7 96.3 97.0 97.9 97.4 1.22

%  difference in average
dose

PTV -1.82 -1.27 -0.42 -0.83 -1.29 -1.52 -0.16 -0.60 -1.49 0.82 -0.86 0.79
Rectum 0.43 0.50 1.64 2.35 0.40 0.18 3.49 0.39 -3.45 1.13 0.71 1.81
Bladder -3.75 -1.97 0.02 -2.03 -1.93 -1.63 -2.38 -2.61 1.06 0.78 -1.44 1.56
Bowel -3.19 -0.68 13.1 15.5 0.16 -0.53 17.6 3.34 -0.87 -2.46 4.20 7.99

average 3D gamma
PTV 0.50 0.40 0.18 0.28 0.41 0.44 0.42 0.26 0.33 0.33 0.36 0.10

Rectum 0.33 0.27 0.20 0.21 0.34 0.14 0.27 0.15 0.12 0.27 0.23 0.08
Bladder 0.40 0.34 0.12 0.24 0.41 0.26 0.23 0.18 0.06 0.23 0.25 0.11
Bowel 0.28 0.23 0.11 0.18 0.26 0.18 0.2 0.25 0.19 0.20 0.21 0.05

%  difference in D95 PTV 0.34 -0.41 0.23 0.07 -0.20 -0.64 -2.43 -0.78 -0.43 -1.43 -0.57 0.84

VMAT plans

In Table 2, 10 H&N patients percentage difference of average
dose, percentage difference of D95 and D1 and average 3D
gamma between Eclipse TPS calculated and COMPASS
measured for PTV's and critical OAR's were listed. Also,
percentage variation of point dose measurement and GAI
was listed. Table 3 summarizes the percentage difference of
average dose, percentage difference of D95 and average 3D
gamma for PTV, rectum, bladder and bowel for 10 pelvis
patients.

Discussion
Advantage of COMPASS 3D dosimetry system over other
QA systems is its capability of performing 3D dose recon-
struction on patient CT scan using beam modeling, detector

measurement and treatment plan. The dose calculation algo-
rithm in an inhomogeneous medium was completely differ-
ent in Eclipse TPS and COMPASS 3D dosimetry system. The
COMPASS uses CCC/S algorithm, where the dose calculation
is based primarily on a point source dose spread array. The
Eclipse TPS uses AAA wheredose calculation is based on a
pencil beam in association with lateral density scaling. In
CCC/S, the dose at a point from a point source of given
TERMA (total energy released per unit mass) to the dose at
another location in a patient can be calculated by scaling
both primary and scatter. Point to point density scaling of
this kind is not feasible by the pencil beam kernel method.
The point spread kernel based method allows greater flexi-
bility in dealing with 3D inhomogeneous medium than pen-
cil beam kernel. Due to this, maximum average dose differ-
ence of 3.2% in air was observed.14, 20 Open field measure-
ments provide vital information for fine tuning of beam



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modeling and help to understand the efficiency of dose re-
construction, especially in the buildup, air and penumbra
region, so that one can set the tolerance level when OAR
falls in region of interest. For example, in H&N patients lips
lie in surface (build-up), trachea lies in air and cochlea lies in
the field border (penumbra).

For PTV in Table 1, there was a systematic decrease in GAI
and increase in average 3D gamma with respect to increase
in treatment delivery errors. However, 2D planar dosimetry
does not provide any clinically relevant information about
the results,12 whereas COMPASS system provides the signif-
icance of error in PTV and as well as in OAR’s. The infor-
mation provided by traditional 1D point dosimetry and 2D
planar devices cannot easily be translated onto dose devia-
tions in the tumor and/or at OAR's. DVH based evaluation
will be a good alternative since it allows physicist and physi-
cian to accept or reject the treatment plan based on the dose
difference in PTV and OAR's. Measurements based dose
reconstruction of COMPASS enables us to verify the treat-
ment delivery with acceptable accuracy (±5%). PTV in Table
1 shows there was a systematic increase and decrease in the
percentage difference of average dose with respect to known
dose delivery errors, and an increase in average 3D gamma
with planned MLC errors.16,21 In the COMPASS system, the
difference between predicted and measured response is into
two components.16 The first component is a scaling correc-
tion (Acorr) factor, which is used to find dose errors (incorrect
MU) and the second component is the remaining residual
response, which is used to reconstruct MLC errors. The cal-
culation grid resolution used in the TPS and COMPASS af-
fects the accuracy of the dose distribution calculated. Litera-
tures have reported that a 2.5 mm isotropic grid produces an
accuracy of about 1% in the high-dose gradient region of an
IMRT plan consisting of multiple fields.23 Benedict et al.23
and Park et al.24 have recommend grid size less than 3 mm
for stereotactic treatments. In both Eclipse TPS and COM-
PASS QA system grid size resolution can be varied from 1
mm to 5 mm, for our study we have chosen 2.5 mm grid size.
Although smaller grid size can yield more accurate results,
2.5 mm grid size was chosen by optimizing the accuracy and
computational time.

The results show that 3D dose parameters were well within
the clinically acceptable tolerance level of ±5%.22 The aver-
age 3D gamma for PTV's and OAR’s for twenty patients used
in this study were less than the recommended value of 0.6 by
Visser et al.17 In three pelvis cases due to low dose of bowel,
the percentage difference of average dose was more than
10%. DVH in COMPASS provides many statistical tools;
however, we cannot judge plan quality by choosing one cri-
teria, as the percentage difference in average dose, dose at
volume and volume at dose will be too high for structure
lying in low dose region. For higher deviation of fluence
over small distances there are limitations due to the recon-
struction capabilities of COMPASS and chamber resolution

of MatriXXEvolution. So COMPASS may slightly underestimate
and/oroverestimate the actual delivered dose for OAR's.14
Despite local inaccuracies in the dose reconstruction, Godart
et al.16 have proven that COMPASS can be used to perform
pretreatment verification of IMRT treatment plans. COM-
PASS can compute dose in patient CT scan using only inbuilt
beam model (without measurements). This model based dose
calculation, function as an independent secondary TPS veri-
fication. COMPASS have dose calculation engine (CCC/S)
similar to TPS, it not only verify the plan parameters (MU)
but also provides anatomically localized QA dose infor-
mation. Kunnanchath et al.25 have showed good agreement
in comparison of Eclipse TPS calculated vs COMPASS calcu-
lated with average dose difference less than 1% and average
gamma less than 0.5 for PTV and OAR’s of 10 head and neck
and 10 pelvis IMRT plans. Previous studies have shown that
measurement (MatriXXEvolution) based dose reconstruction in
phantom images were in excellent agreement with the ion
chamber results and planar dosimetry (film).13-15 Our study
shows that COMPASS is a better system than point and 2D
planar dosimetry by providing the 3D dose discrepancies in
the region of interest in patient anatomy.

Conclusion
Traditionally QA methods are performed in a phantom and
it is often difficult to quantify and interpret the results in
patient anatomy. The ability of the COMPASS software to
reconstruct the 3D dose distribution on patient CT from the
measurements, provides a unique perspective for medical
physicist and radiation oncologist to evaluate the patient’s
QA plan. The results from this study show that the COM-
PASS along with MatriXXEvolution can be effectively used for
pretreatment verification of VMAT plans and it is good
enough to find clinical significant errors in dose delivery.

Conflict of interest
The authors declare that they have no conflicts of interest.
The authors alone are responsible for the content and writ-
ing of the paper.

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