Introduction
Radiation Therapy plays an important role in cancer
treatment. The treatment techniques, such as Intensity
Modulated Radiation Therapy (IMRT), Volumetric Arc Therapy (VMAT) and Stereotactic Radiation
Therapy (SRS), with advancing technology, enable
complex dose distribution around target volume with
minimal damage to normal tissues.[] These techniques
require a series of irradiation fields of varying intensity for implementing the prescribed dose to the
target volume.[-]
The major challenges with the advancement of
technology in transitioning from three-dimensional
conformal therapy to IMRT are the lack of error data
with the new treatment techniques and the loss of applicability
of traditionally used methods and dosimetric
equipment. For example, in addition to the point
dose measurements used in conformal treatment, the
two-dimensional dose mapping has been added.[,]
In this context, the European Society for Radiotherapy
& Oncology (ESTRO) has launched a network of
Quality Assurance of Intensity Modulated Radiation
Oncology (QUASIMOD) among fifteen European centers.[] In the Task Group 119 report published by
the American Association of Physicists in Medicine
(AAPM), point-dose measurement and planar dose
verification are safe for IMRT application.[] The reason
for this is that the radiation dose planned before
treatment in IMRT should be fully confirmed.[]
The improvements in IMRT procedures have had
a significant impact on both the clinical and physical
development of radiotherapy. Quality Assurance (QA)
procedures and the clinical requirements of IMRT have
been the driving force behind many medical physics
research activities.
There are several dosimetry systems, such as ionization
chambers, GafChromic films and two-dimensional
(2D) arrays, which can be used for quality assurance
before patient treatment. Ionization chambers are used
to compare absorbed doses calculated in the treatment
planning system to the measured on the treatment machine.[] The GafChromic film confirms the planar
dose by 3D irradiation, making the same arrangement
in clinical treatments. Van Esch et al. reported that the
time they spend for all these procedures is 3-10 hours
per patient.[] Thus, the film dosimeter has been relocated
gradually with 2D multiple detectors for pretreatment
verification of patient-specific IMRT dose
distribution because of its ease of use and immediate
evaluation of the results,[] which requires much less
time than is required to perform a similar analysis with
GafChromic film. The 2D diode array is ideal for quality
assurance per plan after an IMRT system is fully
commissioned.[]
The human body has many non-homogeneous
structures and does not have a square and/or rectangular
structure as modeled in studies, such as a 2D array
and/or solid phantoms. In addition, separate measurements
of absorbed dose by ion chamber and dose distribution
map by 2D array lead to time loss.
In treatment planning systems (TPS), uncertainties
in dose calculation are a result of dose and position
errors when the dose calculation is interpolated
linearly between grid points. Niemierko and Goitien
have shown that the magnitude of dose and position errors
depends on the width of the beam penumbra.[]
Although a smaller grid size can give us a more accurate
and consistent dose calculation in size, especially in
high dose gradient regions, a finer calculation requires
a longer calculation time than dose calculations using
grid size.[,] Several studies have been conducted
looking at the grid size for different treatment fields and
techniques. Dempsey et al. found that the 0.25 cm grid
size range was sufficient to reduce the dose calculation
error in IMRT to less than 1% using Fourier analysis.
[] Variable grid size with 0.2-0.4 cm differences was
found to cause a 2-4% dose mismatch for head and
neck IMRT.[] The optimal grid size was found to be
0.3 cm in lung stereotactic therapies using dynamic
arc therapy.[] While there are many studies for the
grid size for the calculation of patient plans, there is no
clear answer for the selection of grid size to be used in
calculations for patient QA plan before treatment. In
patient QA processes, tools, such as film, 2D array and
ion chamber, are used. In these measurements, we think
that the effect of grid size is more important in the results
obtained with the 2D array. The reason we think
this may be important is that because the distance of the
ion chambers in the 2D array systems is larger, which
requires interpolation between points.
The gamma evaluation (γ) is the examination of the
plan according to both successful and unsuccessful criteria
for dose distribution comparisons on the base of
both dose difference (DD) and distance to agreement
(DTA). The criterion of success is that the gamma value
is equal to and/or less than 1.[] During gamma evaluation,
the dose change for the distance between the
two selected points is checked. Even though the dose
difference of 3% criterion is set to be the standard for
the dose difference, it becomes important to choose the
dose between two points in the selection of DTA.[]
The main reason for that is DTA"s sensitivity in low
dose gradient regions. For plan acceptance, the percentage
value of the rate that meets this criterion may vary
depending on the intra-clinical decision. Pulliam et al.
reported QA results for 13.000 patients receiving IMRT
in their clinics that the most point dose differences were
within ±3% of tolerance, whereas failed plans were only
a few % beyond tolerance.[] Furthermore, in gamma
evaluations, the initiation criterion of IMRT in their institution
was 90% of the pixels that exceeded the 5%/3 mm
criterion without the low dose threshold.[] In our
clinic, the acceptance criterion was determined as 98.5%
of the pixels exceeding 3%/3 mm criteria.
In this study, a setup that can be used for both absolute
dose and planar dose distribution is considered
aimed to answer the following questions:
1. What is the effect of the calculation grid size value
selected in the TPS on the Monitor Unit (MU)?
2. What is the effect of the selected grid size value in
the TPS on 2D array gamma evaluation and absorbed
dose measurements?
3. What is the effect of using different DTA values on
gamma result in 2D array measurements?
4. What is the effect of using cylindrical/inhomogeneous
(C/I) phantom instead of square/homogeneous
(S/H) on 2D array gamma evaluation and
absorbed dose measurements?
Methods
Preparation of Phantoms and CT Examination
Two different phantom assemblies schematically illustrated
in Figure 1. In both embodiments, the CC04
(IBA Dosimetry GmbH, Germany) model ionization
chamber with an active volume of 0.04 cc on the 2D
array (I"mRTMatriXX of ScanditronixWellhofer, Germany),
consisting of 1020 (detectors with an active volume of 0.08 cc in diameter and a distance between the
detectors of 7.62 mm and a diameter of 4.5 mm, can
measure 2D dose map array which is called MatriXX.
Fig 1: Schematic representation of the phantom assemblies prepared for Setup 1 and Setup 2.
SDD: Source detector distance
In the first embodiment, there are 20 water-equivalent
solid phantoms (SP34 white polystyrene IBA
Dosimetry GmbH, Germany) for S/H, and each size
is 30cmx30cm x1cm (Setup 1). In the second embodiment,
instead of all S/H water-equivalent phantoms
over the ion chamber, a cylindrical/inhomogeneous
phantom of a 20 cm diameter, which includes nine
materials of different density (Setup 2) was placed (By
Truck The phantom laboratory, Greenwich, NY).
In both phantom assemblies, General Electric (GE)
brand (GE-Light Speed 64, GE, USA) Computed Tomography
(CT) device, axial cross-section, taken slices
were sent to ElektaXiO (CMS Co., Ltd, St Louis, MO,
USA) treatment planning system.
Procedures in TPS
The phantom frame is contoured at TPS on transverse
sections of two different assemblies from CT. A 7-field
IMRT plan with 6 Mega Volt (MV) photon energy of
a head and neck patient was calculated for three different
grid sizes (0.3, 0.2 and 0.1 mm). All calculations
for IMRT patient QA in TPS were performed using the
superposition algorithm. Superposition algorithm accounts
for scattering component transport in heterogeneous media. Kernels were modified with a relative
electron density of the media. Also, the step and shoot
optimization techniques were used.
When calculating dose and MU data, Q1 (Q1=Field
1) is the first field irradiation. Consecutive fields are
expressed as the sum of the dose values. For example;
the second field is Q2=Q1+Field2, while the last field is
calculated as the sum of all fields Q6+Field 7 (Q7). At
the same time, the dose administered per field is taken
as the reference for 0.2 cm grid size and the MUs corresponding
to this dose in the other grid sizes are calculated
according to the same formulation.
For MatriXX measurements, the 2D plane dose
maps at the point where the detectors are located were
calculated for each grid size and both phantom assemblies.
The calculated and measured values were compared
by Gamma Evaluation Method.
Procedures on Linear Accelerator Measurements
Before starting the measurements, the Linear Accelerator
(Elekta Synergy Platform, Elekta, Crawley, UK) was
calibrated at a maximum dose point of 6 MV, equivalent
to 1 cGy to 1 MU on 10x10 cm2 field size and 100
cm source-surface distance.
The phantoms are positioned in the geometry and
position placed on the CT on the treatment table. In order to check the placement of the phantoms and the
accuracy of the planned isocenter in the TPS, the Anterior-
Posterior and Lateral images of both phantom
assemblies were taken before the measurement, and
necessary geometric corrections were made.
In both assemblies, readings in the ion chamber
were taken with the Dose1 Electrometer (Dose 1 Electrometer,
IBA Dosimetry GmbH, Germany). The readings
were converted to absorbed dose using calibration
factors and temperature-pressure correction.
Source to MatriXX Detector plane distance (SDD) at
100 cm where ion chamber placed 1 cm above detector
plane and simultaneous measurements were taken. Measurements
were compared with calculated plans made
on different grid size. The planar dose map measured in
the comparison is converted according to the grid size
selected in the planning. In the gamma evaluation, the
dose difference was always taken as 3%, while for DTA, 2
mm and 1 mm, two different distances were used.
Results
While the defined dose is constant, the MU values in
seven sub-fields for IMRT planning according to grid
size change are found for Setup 1 and Setup 2. As shown
in Figure 2, Setup 1 defined doses are used as a grid size 0.2 cm reference and taken as "1". For all fields, the
MUs were normalized to the reference value. According
to the results, while Q1, Q2 and Q3 were decreased by
grid size, MU increased by Q4, Q5, Q6 and Q7 and vice
versa. The largest MU difference was found to be 0.81%
(122.8-123.8) and 1.2% (121.9 -123.4) for Setup 1 and
Setup 2, respectively, between 0.3 and 0.1 cm. The biggest
difference between Setup 1 and Setup 2 was 0.3 cm
for the grid size, which corresponds to 0.73% difference.
Fig 2: After a grid size (in this case 0.2 cm) value was set to 1, the other values were normalized accordingly and MU
variations were graphed against the Q1 to Q7 IMRT fields for Setup1.
The absorbed dose measurements taken in the ion
chamber and the TPS dose calculations for different
grid sizes are shown in Table 1 for Setup 1 and Setup
2. The considerable difference between the calculated
dose (CD) and measured dose (MD) for grid size
0.1cm was found to be 2.96% for the square/homogeneous
phantom and 5.75% for the cylindrical/inhomogeneous
phantom. Although the percentage difference
of CD-MD according to the grid size differs for
each field, the Q7 total measurement evaluation shows
that the best 0.3 cm grid size for square/homogeneous
phantom and 0.2 cm grid size for cylindrical/inhomogeneous
phantom.
Table 1: In both setups, for all field and grid sizes, the absorbed dose values calculated in the TPS and measured by the ion
chamber and the % dose differences between them
Also, as one can see in Table 1, the difference between
the measured and calculated absorbed dose
comparisons was found to be higher in the presence of
cylindrical/inhomogeneous phantoms. The biggest differences
in the Q1 field CD-MD comparison, % dose
differences for Setup 1 were found to be 5.36 for 0.1 cm
grid size and for Setup 2 were found to be 6.73 for 0.3
cm grid size (Table 1).
The results of gamma evaluation of the measurements
taken by MatriXX according to different grid
sizes are shown in Table 2 for Setup 1 and Setup 2. In
this study, 99.5% gamma value was taken as reference
for 3%/3 mm found according to the plan calculated
with 0.2 cm. According to the results, gamma evaluation
in Setup 1 provided γ≤1condition better than
Setup 2 for all grid sizes and DTA values.
Table 2: The results of the gamma evaluation of the measured values by MatriXX and calculated differences % between
reference values (99.5%) and obtained values at different grid sizes
In our clinic, this plan, which was found to be successful
in routine patient QA evaluation, achieved the
best value in the use of cylindrical/inhomogeneous
phantom using the 3%/1mm (94.25). According to the
3%/2 mm criterion we evaluated in the clinic, Setup 1
allowed treatment with 98.76%, whereas this value was below our acceptance limit with 88.39% for Setup 2.
The percentage differences of the other gamma values
according to the reference value were calculated and
given in Table 2.
Discussion
This section has been devoted to answering the questions
posed in the previous sections.
1. What is the effect of the calculation grid size
value selected in TPS on MU?
The MU and segment sizes significantly affect radiotherapy
quality, as well as cancer risk that may arise
from radiation therapy [-], which is also effective
in the development of secondary cancer with an increase
in MU.[,] In a plan equivalent to the human
body with an inhomogeneous structure, Park et
al. found the increase in MU with grid size change, 1%
for 0.3 cm and 2% for 0.4 cm.[]
In our study, when the 0.2 cm grid size for Setup
1, including square/homogenous phantom, was taken
as a reference grid size, the MU values for 0.3 cm and
0.1 cm increased by 0.09% and 0.22%, respectively. For
Setup 2 containing cylindrical/inhomogeneous phantoms,
MU values for 0.1cm increased by 0.12%, and
decreased by 0.15% for 0.3 cm. In the evaluation for
critical organs, Dempsey et al. also reported that when
the higher grid size was studied, MU would be more
likely to cause overdose, especially in series and critical
organs, and the planned target volume would provide
lower coverage.[]
2. What is the effect of the selected Grid Size value
in the TPS on 2D array gamma evaluation and absorbed
dose measurements?
IMRT plans calculated in large grid sizes show lower
doses in high gradient dose regions.[] In our study,
similar results were found for ion chamber placement, which is known to coincide with the high gradient dose
region. The findings showed that there are differences in
the grid size selection in the assessment of calculatedmeasured
dose differences per field and dose difference
assessment for the total field. This may be a separate research
topic, and all CD-MD differences in Setup 1 appear
to be within limits (<3%). In Setup 2, the dose differences
for all grid size calculations, results are within acceptable
limits (<3%) except the Q1 field. The distance between
MatriXX detectors is 0.762 cm. Therefore, the software
interpolates existing doses at intermediate values. In the
Niemierko and Goitein"s studies, the grid evaluated the
accuracy of interpolated doses using a Fermi function
that provides a one-dimensional high-gradient dose
profile for linear interpolation and beam penumbra according
to size.[] The beam penumbra region showed
a steeper dose drop due to lateral electron disequilibrium
when small fields were irradiated.[] Therefore, the use
of large grid sizes in high-dose gradient regions will increase
the error even more since electron disequilibrium
is more effective, especially in small fields.
Our clinical standard is to establish the patient plans
calculated in the 0.2 cm grid size and the QA plan with
0.1 cm and to perform gamma evaluation according to
the 3%/3mm criteria after measurement. For grid sizes
used in this study, the best results for all three evaluation
criteria in Setup 1 were found in grid size 0.2. For
Setup 2, the grid sizes give us more concordance between
CD-MD and the results in 0.1.
3. What is the effect of using different DTA values
on gamma results in 2D array measurements?
Pathak et al. found that the 5%/5 mm evaluation
criteria exceed 95% of all grid size calculations. In the
same study, it was reported that as the DTA shrinks,
the passing rate falls below 90%.[] In our study,
when we look at the differences according to the reference
gamma pass value in Table 2, this value decreases as DTA decreases for both setup and all grid sizes and
increases the difference with the reference value. The
largest difference for Setup 1 and Setup 2 was found to
be 5.28% and 15.43%, respectively, at grid size 0.1 cm
and 0.2 cm for 1mm DTA.
4. What is the effect of using cylindrical/inhomogeneous
(C/I) phantom instead of square/homogeneous
(S/H) phantom in 2D array gamma evaluation and absorbed
dose measurements?
For absorbed dose measurements with ion chamber
and TPS absorbed dose calculations, when the Setup 1
and Setup 2 were compared, the most appropriate grid
for the measurements with S/H phantom was found to
be 0.3 cm and 0.2 cm for Setup 2. The biggest differences
in grid size were found to be 1% for 0.3 cm and
1.52% for 0.2 cm. This shows the effects of inhomogeneous
environment and cylindrical phantom on the
dose change in absorbed dose measurements.
In the literature, a study with clear information
on dosimetric comparisons of C/I phantoms used for
patient QA uptake and S/H phantoms used for standard
QA uptake, to our knowledge, was not available.
In this study, the environment was transformed into a
C/I structure, and values below the acceptance gamma
criteria were encountered.
There are nine inserts of different density in the
phantom, which contains inhomogeneous structure.
The behavior of photons in a medium of homogeneous
density is not the same in environments with different
densities. The reasons for this are photon interactions
with materials with different components, mass reduction
coefficient, effective atomic number and electron
density parameters. The mass reduction coefficient
(µ/ρ) is a measure of the number of photons that interact
(scatter/absorber) with the target material. When a
photon beam passes through the material and interacts,
the photons are either absorbed (photoelectric effect,
double and triple formation, photonuclear) or scattered
(coherent and incoherent scattering).[] Water equivalent
solid even though the dominant phenomenon in
homogeneous phantom is again the incoherent scattering
when the photon beam becomes a bone structure, it
is seen that the coherent scattering increases compared
to water, and especially the photoelectric event increases
more than five times. In the same way, nuclear pairing
increased, and electron pairing decreased.
Conclusion
A setup where both the ion chamber and MatriXX
were used simultaneously in the patient QA test before treatment with IMRT was proposed in this study. The
use of this setup will save time for the medical physicist.
Moreover, the gamma evaluation results changed
significantly using C/I phantoms containing inhomogeneous
structures in place of routinely used the S/H
phantoms. While the grid size was 0.2 in Setup 2, the
decrease rate in gamma evaluation was found higher
than 0.1 grid size value. In absorbed dose comparisons,
the differences between homogeneous and inhomogeneous
phantom were found to be less than 3%.
Peer-review: Externally peer-reviewed.
Conflict of Interest: None declared.
Ethics Committee Approval: This study was conducted in
accordance with local ethical rules.
Financial Support: None declared.
Authorship contributions: Concept - A.İ.; Design - A.İ.,
İ.H.S.; Supervision - A.İ., İ.H.S.; Funding ? None; Materials
- A.İ., İ.H.S.; Data collection and/or processing - A.İ.; Data
analysis and/or interpretation - A.İ., İ.H.S.; Literature search
- A.İ, İ.H.S., R.Ü.; Writing - A.İ., İ.H.S., R.Ü.; Critical review
- A.İ., İ.H.S., R.Ü.
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