Introduction
In megavoltage photon beams, the electron contamination
of the incident beam causes surface dose, which
is defined at the boundary between air and patient. In external beam radiation therapy, accurate knowledge
of surface dose may help reduce the risks of acute skin
reactions and delayed effects such as erythema, necrosis,
desquamation, and dermal lymphatic and basal-cell
carcinoma.[] Also, it is important to know the surface dose in the irradiation of superficial tissues to ensure
that the target receives the prescribed dose. Surface dose
is mainly contributed by the scattered radiation from
materials in the path of the beam, air, and patient.[]
The energy of the beam, the size of the irradiation field,
and the source to skin distance are the parameters that
directly affect surface dose. The oblique beam incidence
and use of beam modifiers such as immobilization devices,
bolus, and block tray increase the deposited dose
at shallow depths. In most cases, megavoltage photon
beams are utilized in radiotherapy. Due to the lack of
electron equilibrium, the use of high-energy photon
beams result in lower deposited dose at the surface.[]
This steep dose reduction in the surface area is known as
the skin-sparing effect. Accurate knowledge of the dose
at such shallow depths is required in special measurement
techniques and devices.
The measurements of surface and buildup region
doses are difficult. Extrapolation ionization chamber
is the most accurate dosimetric device to measure
doses at shallow depths, but not all institutions have
this equipment. Due to their thin entrance window, the
fixed-separation parallel plate ionization chambers can
be used for surface and buildup region dose measurements
instead of extrapolation chamber, but secondary
electrons scattering from the sidewall of the chambers
cause overresponse. This problem can be solved using
Gerbi"s correction factors.[] These chambers can only
be used with phantom measurements because of their
physical geometry. Radiochromic film is an appropriate
dosimeter for surface and buildup region dose measurements
with its high spatial resolution and low spectral
sensitivity. The characteristics of radiochromic film
make it a substantial dosimeter for the regions of steep
dose reductions and also make it a good alternative to
parallel plate ionization chamber. Bilge et al. [] utilized
EBT2 radiochromic film for surface dose measurements
and compared the results with those of a parallel plate
ionization chamber. The difference between EBT2 radiochromic
film and ionization chamber was found to
be within 5% and 3% for 6 MV and 18 MV, respectively.
Also, the physical properties of radiochromic films allow
in vivo dose measurements.
Treatment planning systems (TPS) use algorithms
to calculate dose distributions in irradiation area. Accurate
dose calculation for surface and buildup region is
a challenge for most commercial algorithms. The dose
prediction of TPS at surface and buildup region depends
on several factors including calculation algorithm, beam
modeling, and linac commissioning. Contaminated electrons
induced from a collimator system and secondary photons scattering from the linac head cause contribution
to the dose. This contribution is the main reason for
the difficulty of dose calculation at superficial regions for
algorithms.[] Previous studies have reported that some
commercial TPS underestimate surface and buildup region
doses by 10%?30%.[,] Calculated doses near the
surface obtained by TPS are generally inaccurate due to
the lack of electronic equilibrium.[]
In radiation therapy, surface dose calculation accuracy
is important in cases when the target volume is close
to the skin. The precision of TPS should be assessed to
prevent potential risks of toxicity and to provide acceptable
dose coverage to target volumes near the surface.
In this study, our aim is to investigate surface dose with
radiochromic film measurement and TPS calculation
for intensity-modulated radiotherapy (IMRT) and volumetric
modulated arc therapy (VMAT).
Methods
Phantom
A tissue-equivalent anthropomorphic Rando phantom
(Alderson Research Laboratories, Stanford, CT ) was
used for irradiation. Computed tomography (CT) images
of the phantom were acquired with head-first supine
position using Philips Brilliance Big Bore CT (Philips
Healthcare, Cleveland, OH) (Fig. 1). Thermoplastic mask
was utilized for head and neck immobilization as it is used
for real patients. Slice thickness of 3 mm was chosen for
CT scanning. Then, images were sent to TPS to determine
the surface dose for IMRT and VMAT techniques.
Fig 1: CT images of tissue-equivalent anthropomorphic
Rando phantom.
Planning
In this study, CT images of 5 larynx cancer patients were
used to create IMRT and VMAT plans. Slice thickness of
3 mm was chosen for CT images. Targets and organs at
risk volumes were defined and contoured by a radiation oncologist. Treatment plans were created using Eclipse 8.9
(Varian, Palo Alto, CA, USA) TPS on Varian Trilogy linear
accelerator equipped with a 120-leaf MLC, performing
6-MV coplanar photon beams. Analytical anisotropic
algorithm (AAA) was utilized for dose calculation. A calculation
grid of 2.5 mm was chosen for treatment plans.
Doses of 70 and 54 Gy were prescribed for target volumes,
and simultaneous integrated boost method was used for
each plan with a fraction number of 35. The optimization
aim of all plans was that 95% of the target volumes should
receive 100% of the prescribed doses.
IMRT plans were generated with 7 coplanar fields,
which were separated at 52° apart. Sliding window technique
was chosen. A fixed dose rate of 500 MU/min was
applied for dose delivery. VMAT plans were created using
a dose rate of 600 MU/min, which is the maximum
value of the linac. VMAT plans consisted of 2 full arcs
rotating from 179.9° to 180.1° counter clockwise and
clockwise. Collimator angles were fixed to 30° and 330°
to avoid tongue and groove effect. Then quality assurance
(QA) plans of both IMRT and VMAT techniques
were generated using Rando phantom for irradiations
and measurements (Fig. 2). The isocenter of the plans
and positions of gantry, collimator, and couch were set
to the same as those in plans.
Fig 2: QA plans of IMRT (a) and VMAT (b) techniques
on Rando phantom.
Film dosimetry
Gafchromic EBT3 film (International Specialty Product,
NJ, USA), which has a single active layer of approximately
30 ?m thickness sandwiched between two
125-µm transparent polyester sheets, was utilized in this
study. Compared with older versions of radiochromic
films, EBT3 is more sensitive with its wide dose range of
1 cGy to 40 Gy and has symmetric structure that allows
scanning on either side.
Before measurements, a calibration curve was created
for the film batch. The films cut into 2.5×2.5 cm2 and
placed perpendicularly between the water equivalent
slab phantoms at the depth where the linac calibrated 1 cGy equals to 1 MU. Films were irradiated at 0?800 cGy
at a field size of 10×10 cm2. Unirradiated film piece was
used as background. The net optic densities (ODs) of the
irradiated films were corrected to the known doses to
create the calibration curve, which was used for converting
net ODs to absolute doses in measurements.
EBT3 films were put on the surface of the center
of the larynx (Fig. 3). Irradiation was performed with
treatment fields of IMRT and VMAT plans using 6-MV
photons beams. Films were scanned 24 hours after irradiation.
The calibration of the film batch was used to
acquire absolute doses.
Fig 3: EBT3 film placement on Rando phantom for
measurement.
TPS calculation
In this study, AAA was performed for treatment plan
calculations. AAA has 3 source models including primary
photons, scattered extra-focal photons, and contaminated
electrons from beam-limiting devices such as
collimators. The algorithm accounts the heterogeneities
anisotropically using lateral photon scatter kernels. The
electrons generated through Compton scattering from
the linac head and air were modeled by the electron
contamination source, which utilizes a depth-dependent
curve defining the dose from lateral electron contamination.
This source has a major role in surface and buildup
region dose calculation. The dose distributions are acquired
by superposition of doses from electron and photon
convolutions in AAA.[,]
The location of the point which was on the surface
of larynx used in film measurements was determined
for IMRT and VMAT plans in TPS. Then, surface doses
calculated using AAA were obtained from TPS. The results
of the calculations were then compared with film
measurements.
Open field irradiation
A comparison between film measurements and TPS
calculations was also made for open field. Surface and buildup region doses were measured and calculated in
a field size of 10×10 cm2 using water equivalent slab
phantoms for 6-MV photon beams. A phantom set with
a thickness of 15 cm was prepared for CT scanning. CT
images were sent to TPS for open field calculation. The
same phantom set with 2.5×2.5 cm2 film piece on the top
of it was irradiated under the same conditions as in the
TPS. The results were then compared.
Results
The results and differences of the Eclipse TPS calculations
and Gafchromic EBT3 film measurements for surface
dose in IMRT and VMAT techniques are shown in
Table 1 and Table 2, respectively. Surface doses obtained
from film measurements were found to be higher in
IMRT plans compared with TPS calculations with an average
difference of 12.9%, and the median of the difference
was 15.0%. Surface doses acquired from film measurements
were also found to be higher in VMAT plans
than in the TPS calculation with an average difference of
12.4%, and the median of the difference was 14.3%.
Table 1: The results and differences of TPS calculations
and film measurements for surface dose in
IMRT technique
Table 2: The results and differences of TPS calculations
and film measurements for surface dose in
VMAT technique
Statistical Package for the Social Sciences version
11.0 software (IBM, Chicago, IL, USA) for Windows was
performed for statistical data management and analysis.
The Wilcoxon signed-rank test was applied to determine
statistical significance with p-values of <0.05 considered
to be significant. The surface dose differences between
TPS calculations and film measurements for IMRT and
VMAT techniques were found to be statistically significant.
The comparison of TPS calculation and EBT3 film
measurements for surface and buildup region doses at
10×10 cm2 open field at 0, 1, 2, 5, 10, and 15 mm were
made to evaluate the results of IMRT and VMAT techniques.
The results are given in Table 3. The obtained
PDDs show that doses calculated by TPS are lower than
film measurements at shallow depths.
Table 3: Percentage depth doses (%DDs) of 6-MV photon
beam for 10×10 cm2 open field
Discussion
Dose measurement at the surface and buildup region
is challenging. The physical properties of dosimeters
make surface dose measurements more difficult. Accurate
knowledge of doses near surface area is helpful
in making clinical decisions such as determining the
prescribed dose, especially in cases where the skin is
defined as a target or a dose-limiting volume. Acute
reactions and delayed toxicities can be prevented by
avoiding overirradiation of the skin. The chosen energy
for irradiation, size of the field, obliquity of the beam, and complexity of the planning directly affect the
surface dose. These parameters also affect the ratio of
electron contamination, which has a major role in surface
dose occurring and is difficult to calculate with TPS.
Intensity-modulated techniques such as IMRT and
VMAT are commonly used in head and neck cancer
radiation therapy. In this study, 7-field IMRT and double-
arc VMAT plans were generated for 5 larynx cancer
patients in Eclipse TPS using AAA, and QA plans
were created using Rando phantom for film measurements.
The comparison between measurement and
calculation was also made at 10×10 cm2 open field using
water equivalent slab phantoms. Surface dose was
found to be lower in TPS calculations for both IMRT
and VMAT techniques compared with radiochromic
film measurements. The average difference between
the calculations and measurements were found to be
12.9±6.1% and 12.4±7.7% for IMRT and VMAT, respectively.
The open field measurement and calculation
gave closer results for the surface and buildup region
dose in the first few millimeters of the phantom. It is
assumed that the complexity of the treatment fields
used in IMRT and VMAT increased the difference between
measurement and calculation.
It has been reported that the Eclipse calculation for
doses near surface area has a deviation of ±20% in 95%
of all measurement points.[] Akino et al. [] compared
the radiochromic film measurements with AAA
calculations for different radiotherapy techniques including
IMRT in breast cancer treatments and found
that the calculation algorithm underestimated the doses
near the surface by 15%?30%. They also reported
that the et al. [] pointed out that an increase in dose
uniformity might result in the reduction of surface
dose. Another study explained that the dose distributed
around the target causes lower doses at the surface.[]
Almberg et al. [] investigated superficial doses for
conventional tangential and intensity-modulated techniques
and found that the surface dose was reduced
by 20% in multiple-field IMRT technique. The results
of our study are consistent with those of the literature;
surface doses were found to be lower in VMAT than in
IMRT, although some investigators found more accurate
results with AAA compared with older algorithms
such as pencil beam convolution.[,] Also, Chakarova
et al. [] studied superficial dose distribution in
breast radiotherapy using tangential beams. They evaluated
the Eclipse algorithms performing Monte Carlo
calculation and reported that the AAA calculation and
Monte Carlo calculation showed agreement within 3%
dose/4 mm spatial tolerance.
Polednik et al. [] investigated various TPS and
found that the collapsed-cone algorithm underestimates
the surface dose by 20%. Chow et al. [] studied the
accuracy of superposition/convolution algorithm for
oblique photon beams using Monte Carlo calculation
and reported that AAA and collapsed-cone algorithms
cannot accurately calculate the dose below 2 mm. In
our study, dose measurements for IMRT and VMAT
were made at the surface of the phantom and TPS underestimated
the dose by up to 22.2%. Akino et al. []
explained that the dose calculation accuracy at shallow
depths depend on calculation grid size. In their study,
1×1 mm2 grid size improved the dose inaccuracy from
19.1% to 12.0% compared with 2.5×2.5 mm2 grid size
in IMRT treatment plans. We used 2.5×2.5 mm2 grid
size for IMRT and VMAT plan calculation as we used
in clinical practices to evaluate surface doses in our daily
routine.
The uncertainties of Gafchromic EBT films ranging
up to 7% and the heterogeneities occurred from interlot
and intrasheet of Gafchromic EBT films were reported
by previous studies.[,,] The films were utilized
from the same batch and they were evaluated using
the same calibration curve to avoid these uncertainties.
Devic et al. [] measured skin dose for 6-MV photon
beams in clinical applications using Gafchromic dosimetry
films (HS, XR-T, and EBT), and they reported that
even a small thickness of Gafchromic EBT film might
cause an increase in the surface dose. In our previous
study [], Gafchromic EBT3 film gave similar results
with Markus parallel plate ionization chamber, which is
assumed to be the most accurate dosimeter for surface
dose measurements after extrapolation ion chambers. In
this study, film results were used as reference.
The surface doses should be measured for verification
of radiotherapy plan. In this study, the film dosimetry,
which is an appropriate and easy method to
determine the dose at surface, was utilized with Rando
phantom.
Conclusion
Nowadays, in head and neck cancer patient treatment,
higher doses to the target volumes can be delivered using
intensity-modulated techniques such as IMRT and
VMAT. The accurate knowledge of the surface dose is
important to avoid toxicities caused by radiotherapy.
The algorithms of TPS cannot accurately calculate the
surface dose. In our study, surface doses were found to
be lower in TPS calculations compared with film measurements.
The underestimation/overestimation ratio of TPS should be considered while evaluating the radiotherapy
plans. This ratio can be determined by dosimetric
measurements. Radiochromic films are suitable
equipments for this process.
Peer-review: Externally peer-reviewed.
Conflict of Interest: None declared.
Authorship contributions: Concept - U.A., H.B.; Design -
U.A., H.B.; Supervision - U.A., H.B.; Materials - U.A.; Data
collection &/or processing - U.A., N.D.K., C.K., K.Ö.; Analysis
and/or interpretation - U.A., M.A., H.B.; Literature search ?
U.A., N.D.K.; Writing - U.A.; Critical review - U.A., M.A., H.B.
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