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RESEARCH ARTICLE
Characterization of GafChromic EBT2 film
dose measurements using a tissue-equivalent
water phantom for a Theratron
®
Equinox
Cobalt-60 teletherapy machine
Daniel Akwei AddoID
1
*, Elsie Effah KaufmannID
2
, Samuel Nii Tagoe
3,4
, Augustine
Kwame Kyere
5
1Department of Computer Engineering, Kwame Nkrumah University of Science and Technology, Kumasi,
Ghana, 2Department of Biomedical Engineering, School of Engineering Sciences, University of Ghana,
Legon, Accra, Ghana, 3National Radiotherapy Oncology and Nuclear Medicine Centre, Korle-Bu, Accra,
Ghana, 4School of Biomedical and Allied health Sciences, University of Ghana, Accra, Ghana, 5Medical
Physics Department, Graduate School of Nuclear and Allied Sciences, University of Ghana, Atomic, Accra,
Ghana
*danieladdo@knust.edu.gh
Abstract
Purpose
In vivo dosimetry is a quality assurance tool that provides post-treatment measurement of
the absorbed dose as delivered to the patient. This dosimetry compares the prescribed and
measured dose delivered to the target volume. In this study, a tissue-equivalent water phan-
tom provided the simulation of the human environment. The skin and entrance doses were
measured using GafChromic EBT2 film for a Theratron
®
Equinox Cobalt-60 teletherapy
machine.
Methods
We examined the behaviors of unencapsulated films and custom-made film encapsulation.
Films were cut to 1 cm ×1 cm, calibrated, and used to assess skin dose depositions and
entrance dose. We examined the response of the film for variations in field size, source to
skin distance (SSD), gantry angle and wedge angle.
Results
The estimated uncertainty in EBT2 film for absorbed dose measurement in phantom was
±1.72%. Comparison of the measurements of the two film configurations for the various irra-
diation parameters were field size (p = 0.0193, α= 0.05, n = 11), gantry angle (p = 0.0018, α
= 0.05, n = 24), SSD (p = 0.1802, α= 0.05, n = 11) and wedge angle (p = 0.6834, α= 0.05, n
= 4). For a prescribed dose of 200 cGy and at reference conditions (open field 10 cm x 10
cm, SSD = 100 cm, and gantry angle = 0º), the measured skin dose using the encapsulation
material was 70% while that measured with the unencapsulated film was 24%. At reference
irradiation conditions, the measured skin dose using the unencapsulated film was higher for
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OPEN ACCESS
Citation: Addo DA, Kaufmann EE, Tagoe SN, Kyere
AK (2022) Characterization of GafChromic EBT2
film dose measurements using a tissue-equivalent
water phantom for a Theratron
®
Equinox Cobalt-60
teletherapy machine. PLoS ONE 17(8): e0271000.
https://doi.org/10.1371/journal.pone.0271000
Editor: Paula Boaventura, IPATIMUP/i3S,
PORTUGAL
Received: November 11, 2021
Accepted: June 21, 2022
Published: August 19, 2022
Peer Review History: PLOS recognizes the
benefits of transparency in the peer review
process; therefore, we enable the publication of
all of the content of peer review and author
responses alongside final, published articles. The
editorial history of this article is available here:
https://doi.org/10.1371/journal.pone.0271000
Copyright: ©2022 Addo et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
within the manuscript and its Supporting
information files.
open field configurations (24%) than wedged field configurations (19%). Estimation of the
entrance dose using the unencapsulated film was within 3% of the prescribed dose.
Conclusions
GafChromic EBT2 film measurements were significantly affected at larger field sizes and
gantry angles. Furthermore, we determined a high accuracy in entrance dose estimations
using the film.
Introduction
Several types of external photon beam radiotherapy exist [1–5]. Compared to fixed beam
radiotherapy (where there is no modulation in the beam delivered), with intensity-modulated
radiotherapy, modulation of the photon beam involves using multi-leafs collimator and com-
pensators [3,6,7]. Image-guided radiotherapy is another type of external beam radiotherapy;
however, with this technique, the treatment is guided by images obtained from Computed
Tomography or Magnetic Resonance Imaging [5,8–10]. The radiation treatment process
requires the control and assurance of quality in the overall treatment delivery. Several tools are
used to ensure the radiotherapy process is safe and effective. These include port films [11,12];
treatment planning systems (TPS) [13,14]; and phantoms (used to simulate the human envi-
ronment in the provision of scatter) [15–17]. These traditional quality assurance tools are used
before radiotherapy, complicating the detection and rectification of treatment errors [18–20].
In vivo dosimetry is another method for assuring quality in external photon beam radio-
therapy and is performed whilst the patient is receiving treatment [21,22]. Dosimeters placed
in body cavities or on the patient’s skin measure the absorbed doses. In vivo dosimetry may be
employed to detect errors in individual patients [23,24], assess errors in core procedures [25],
evaluate the quality of specific treatment techniques [26] or quantify the dose in cases where
the dose estimation is inaccurate or difficult to calculate [27]. This mode of dosimetry involves
the measurement of two types of absorbed doses. The entrance dose is quantified at the
entrance of the beam while the exit dose refers to the radiation dose estimated at the exit point
of the photon beam.
Different types of in vivo dosimeters are used in external photon beam radiotherapy. In this
research, GafChromic
1
EBT2 film (produced by International Specialty Products, NJ, USA)
was used because of the following characteristics: ease of handling, thin width, good spatial res-
olution, self-developing, nearly tissue equivalent characteristics (with an effective atomic num-
ber of 6.84) [28–30], and wide dose range (between 0.01 and 10 Gy) [29,31,32]. After the film
is irradiated, the active component in the film undergoes a polymerization process resulting in
a color change to blue. Changes in the optical density of the film (resulting from the polymeri-
zation process) are used for absorbed dose quantification.
Dose measurements by EBT2 films are affected by contaminating electrons [33–35]. Several
factors contributing to electron contamination have been investigated in previous studies [36–
39]. Medina, Teijeiro [39] quantified electron contamination and reported that the degree of
electron contamination depends on factors such as photon energy, beam shaper, treatment
field and SSD. They further reported a greater dependence of electron contamination on field
size, SSD, and depth for an 18 MV photon energy when compared with a 6 MV photon [39].
Klevenhagen [38] studied the implication of electron backscatter on electron dosimetry. Their
study concluded that decreasing the treatment field size resulted in a surface dose reduction.
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Funding: The author(s) received no specific
funding for this work.
Competing interests: The authors have declared
that no competing interests exist.
Because of the numerous factors causing electron contamination for film dosimetry, estima-
tion of correction factors can be laborious, and this has the potential of negatively impacting
the accuracy of dose measurements. In this study, we developed a new film setup configuration
whereby the film is "housed" within an encapsulation material with the aim of minimizing
electron contamination. The effects of such film configuration on dose estimations have not
yet been studied. A tissue equivalent water phantom was used in this research to simulate the
human environment. The general aim of this study was to characterize GafChromic EBT2 film
dose measurements using a tissue-equivalent water phantom for a Theratron
1
Equinox
Cobalt-60 Teletherapy Machine. The specific questions that this study sought to address were:
1. How do the two configurations of the film compare in terms of their response to variations
in irradiation parameters?
2. How do variations in irradiation parameters affect skin dose?
3. How accurate are entrance dose calculations by these films?
Materials and methods
We evaluated the behaviors of irradiated films using Theratron
1
Equinox-100 Cobalt-60 tele-
therapy unit with an average photon energy of 1.25 MeV for the following irradiation parame-
ters: field size, SSD, gantry angle and wedge angle. The film used in this study was
manufactured by International Specialty Product inc. with Lot number A06271203. According
to the film’s specifications, the EBT2 film can measure absorbed doses between 1 cGy and 10
Gy. The dimension of the film sheets was 8 cm ×10 cm. We cut each film sheet into 1 cm ×1
cm for calibration, measurement of skin dose and entrance dose estimations. The beam output
from the Equinox-100 Cobalt-60 teletherapy unit was measured using a mini water phantom
(with the dimension 20 cm x 20 cm x 20 cm). The water phantom was made of Perspex and
can accommodate a 0.6 cm
3
farmer type ionization chamber. The ionization chamber used
was a cylindrical farmer chamber type (model PTW30001) manufactured by PTW Freiburg
with serial number 1510 and a volume of 0.6 cm
3
.
We determined the dose rate of the Cobalt-60 unit regularly at reference conditions of field
size (10 cm ×10 cm), SSD (100 cm) and gantry angle (0˚). These reference conditions were
kept constant throughout this study. The calibration of the ionization chamber is traced to the
specifications of the International Atomic Energy Agency’s secondary standard laboratory.
The ionization chamber’s calibration factor (ND, W) was 5.17 and determined for the follow-
ing: a chamber bias voltage of +400 V, the temperature of 20˚C and pressure of 101.325 kPa
for humidity not exceeding 70%. We used the ionization chamber to establish a dosimetric
protocol for the GafChromic film. An electrometer (model PTW UNIDOS with Serial Number
T10005) connected to the ionization chamber was used to quantify the charges created by ioni-
zation. Both the electrometer and ionization chamber were calibrated together for the dosi-
metric procedures. The densitometer used for measuring the optical densities of the films was
manufactured by PTW Freiburg (Germany), called DensiX, and had a serial number of
T52001–3263.
Film preparation
We handled the films according to accredited international protocols [29,40,41]. To use
EBT2 films for in vivo dosimetry, they were cut into squares with the dimension of 1 cm ×1
cm. The films were cut with precaution to minimize scratches and oiling. The same faces of
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the radiochromic films were marked (because of the asymmetry in the film’s structure) to
ensure consistency during radiation exposures. The films were kept in a dark-airtight box to
minimize unnecessary exposure to light and moisture and later used for assessing the effects of
irradiation parameters on skin dose and entrance dose estimations. For each film, the optical
density before irradiation was measured (and recorded as OD1) to correct for background
radiations. The point densitometer was warmed for 5 minutes before being used to increase
the accuracy of the measurements.
Postexposure optical density growth of EBT2 film
Previous studies have shown that optical densities of irradiated EBT2 increase with time [42,
43]. We performed a postexposure optical density growth experimentation to estimate the
optimum time for measuring the optical densities of the irradiated EBT2 films. Four sets of
films, each consisting of three films, were irradiated to doses of 50 cGy, 200 cGy, 400 cGy and
800 cGy. The irradiated films’ optical densities were then measured with the point densitome-
ter at intervals of 60 minutes between 1 minute and 6480 minutes.
Calibration curve for EBT2 film
When radiochromic films are exposed to ionization radiations, the absorbed dose is expressed
through changes in optical density. In radiation dosimetry, it is the absorbed dose which is
required. The changes in optical density are converted to absorbed dose through calibrations.
The films were calibrated using the International Atomic Energy Agency (IAEA) TRS398 pro-
tocol [44,45]. We placed ten sets of films between solid water slabs with the dimension 30
cm ×30 cm ×25 cm and the slabs positioned perpendicularly to the direction of the beam.
The films were at a depth of 5 cm beneath the surface of the slabs. At reference conditions, the
sets of films were irradiated at different treatment times to give absorbed doses of 50 cGy, 100
cGy, 150 cGy, 200 cGy, 250 cGy, 300 cGy, 350 cGy, 400 cGy, 450 cGy and 800 cGy. After irra-
diating the films, they were scanned using the densitometer, and their optical density was
recorded as OD2. We used Eq 1 in estimating the net optical density (NOD). The absorbed
doses determined by ionization chamber readings were plotted against their corresponding
net optical densities to obtain the calibration curve. The calibration curve was subsequently
used to convert any film signal (optical density) to their absorbed dose.
NOD ¼OD2OD1ð1Þ
Where: OD1 is the optical density of the film before irradiation.
Dose uncertainty budget
The dose uncertainties carried out in this study were calculated by error propagation as used
in previous studies [46–48]. Two sources of uncertainties considered during the generation of
the calibration curve were: the experimental and fitting. We assumed the scanning of the films
to be the source of the experimental uncertainties, while the curve fitting uncertainties were
associated with the accuracy of the curve fitting process.
Eq 2 represents a potential equation for the calibration curve:
Dose ¼aOD3þbOD2þcOD þdð2Þ
Where:
OD is the measured optical density; Dose is the estimated dose; a,b,cand dare the
coefficients.
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The final experimental dose uncertainty was calculated by applying Eq 3:
dedose ¼3aOD2þ2bOD þcð Þ dOD ð3Þ
where: δ
e-dose
, is the experimental dose uncertainty; OD, is the optical density measurement;
δ
OD
, is the standard deviation of the densitometer. Since the digital display of the densitometer
is limited to 2 decimal places, the standard deviation of the densitometer was approximated to
0.01.
The curve fitting uncertainty was calculated by applying Eq 4:
dfdose ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
d2
aOD6þd2
bOD4þd2
cOD2þd2
d
qÞ ð4Þ
Where: δ
f−dose
is the fitting uncertainty; δ
a
,δ
b
,δ
c
, & δ
d
are the standard deviation of the fitting
parameters. The standard deviation of the fitting parameters was calculated using the non-lin-
ear model fit function in MATLAB (The Math Works, Inc. MATLAB. Version 2020a).
Finally, the total dose uncertainty was calculated using the Eq 5:
ddose ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðd2
edose þd2
fdose
qÞ ð5Þ
Design of encapsulation cap
We constructed the encapsulation medium (for holding the EBT2 film) using Perspex. To
ensure that the film was measuring the absorbed dose at a depth of electronic equilibrium
(Dmax), a 0.5 cm thickness of encapsulation material was placed above the film. The cross-sec-
tional diameter of the encapsulation material used in the experimentation phase was 4 cm for
easy handling. This diameter is expected to be reduced in actual practice to minimize the
encapsulation material interference in the overall treatment delivery. Fig 1 shows the con-
structed encapsulation cap used in this study.
Entrance dose calibration factor of EBT2 film
Because the absorbed dose measured by EBT2 film is not at Dmax, which ideally quantifies the
entrance dose, calibration of the film was performed for entrance dose measurements. We
determined the entrance dose calibration factor by positioning and irradiating the radiochro-
mic films (at reference conditions) on the surface of a water phantom. Fig 2 shows the setup
for the entrance dose estimations. As illustrated in Fig 2, the ionization chamber was posi-
tioned along the central axis at a distance of 0.5 cm. At this depth beneath the surface of the
phantom, the ionization chamber was probing the percentage depth dose curve at its maxi-
mum dose and not its subsequent fall-off.
Fig 1. Encapsulation cup used for holding the films for radiation dosimetry. The films were positioned at Dmax.
The cup was constructed with Perspex and had a dimension of 4 cm (cross-sectional diameter) x 1 cm (thickness).
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In determining the entrance dose calibration factor, Eq 6 was used.
Fcal;entrance ¼Ricð Þreference conditions
Rfð Þreference conditions ð6Þ
Where: Fcal,entrance is the entrance dose calibration factor
Ric is the reading of the ionization chamber at reference condition
Rf is the reading of EBT2 at reference condition
Correction factors determination when using EBT2 films
Absorbed dose measurements using EBT2 film are highly influenced by contaminating elec-
trons from the air and head of the treatment machine [49,50]. The proportions of these con-
taminating electrons vary depending on the irradiation conditions, hence the need for
correction factors to minimize these interferences and enhance the accuracy of measurements.
In determining the correction factors, we irradiated the films for varying field sizes (4 cm x 4
cm to 24 cm x 24 cm), SSD (75 cm to 120 cm), gantry angle (0ºto 90º) and wedge angle (15ºto
60º). When assessing the effect of each irradiation parameter on the film response, we kept the
other irradiation parameters at reference conditions.
For the same irradiation condition (as performed on the films), the ionization chamber was
used to measure the dose delivered to the water phantom. The data obtained from the irradia-
tions (both the film and ionization chamber) were normalized with measurements obtained at
reference conditions. I.e., The correction factor for each irradiation parameter was calculated
by dividing the ratio of the ionization chamber’s reading to the film’s reading under clinical
conditions by the same ratio under reference irradiation conditions. The equation for
Fig 2. Schematic diagram showing how the calibration of EBT2 film was performed. The films were placed on the
surface of the water phantom, while the ionization chamber was placed at Dmax. The irradiation was performed at
reference conditions.
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estimating the correction factors is shown in Eq 7.
CF ¼
Ric
Rf
clinical condition
Ric
Rf
reference condition ð7Þ
Where: CF is the correction factor
Ric is the reading of the ionization chamber
Rf is the reading of the EBT2 film
Entrance dose calculations
We estimated the entrance dose using the calibration factor, correction factors and the film’s
reading. The entrance doses were determined using Eq 8. The absorbed radiation dose at any
depth within the tissue was determined by multiplying the entrance dose by the percentage
depth dose (PDD) for Cobalt-60 energy.
Dentrance ¼Df Fcal;entrance YCF ð8Þ
Where: Dentrance represents the entrance dose and is the dose at Dmax
Df represents the EBT2 film dose reading
Fcal, entrance represents the entrance dose calibration factor
PCF represents the product of correction factors (for field size, SSD, gantry angle and wedge
angle) used during the dose estimations
Skin dose determination
The skin dose associated with radiotherapy is of interest for clinical evaluation or examination
of the risk of late effects of radiation. The skin dose estimated at a depth of 0.070 mm measures
the amount of radiation deposited in the skin tissue. We quantified the skin dose using two
film configurations (an unencapsulated film and an encapsulated film). For the encapsulated
film configuration, skin dose was determined by placing the film between the encapsulation
material and the skin. Because the depth of the active layer within the film (depth of 0.080
mm) was slightly higher than the required depth for skin dose estimation (0.070 mm), a cali-
bration factor was determined and used to convert the absorbed dose by the film to that of the
skin. In estimating the calibration factor for skin dose estimations, a single EBT2 film was irra-
diated at reference conditions. The optical density of the film was then measured and
recorded. Four films were also irradiated under the same reference conditions. The optical
density of the bottom-placed film was then measured and recorded. The depth of the active
layer of the bottom-placed film was 0.935 mm. The optical density at a depth of 0.070 mm was
estimated using a simple extrapolation technique from the measured optical density for both
the single film and four film setups. The steps involved in determining the skin dose calibra-
tion factor, skin dose and percentage skin dose are shown below.
Dskin ¼Df Fcal;skin ð9Þ
Where: Df is the film’s reading.
Fcal;skin ¼estimated film dose at a depth of 0:07
estimated film dose at a depth of 0:08 ð10Þ
In determining the dose at a depth of 0.07 mm within the film, the extrapolation method
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shown below was used.
0:080 mm 0:03 optical densityð Þ
0:935 mm 0:08 optical densityð Þ
0:070 mm X optical densityð Þ
Therefore :0:935ð Þ 0:080ð Þ
0:935ð Þ 0:070 0:08ð Þ 0:03ð Þ
0:08ð Þ x
Xwas estimated to be 0.03
Therefore X¼0:03 ðoptical densityÞ
Fcal;skin ¼0:03
0:03 ¼1:00
Dose to skin Dskinð Þ ¼ Df 1
%skin dose ¼Dskin
Dentrance 100 ¼Df 1:00
Df Fcal;entrance QCF 100
%skin dose ¼1:00
Fcal;entrance QCF 100
But Fcal,entrance for unencapsulated film was calculated to be 4.134.
Therefore :%skin dose ¼1:00
4:134 QCF 100 24
QCF ð11Þ
Results
Post-irradiation optical density growth
From Fig 3, after irradiating the films, their optical densities increased and gradually became
asymptotic to the horizontal axis with time. Films irradiated with higher doses of radiation
recorded higher optical densities than those exposed to lower doses. Low doses of radiation
(50 cGy, 200cGy) caused the optical density growth to stabilize quickly compared to the films
irradiated with higher doses (400 cGy, 800 cGy).
When the post-exposure optical density growth for an absorbed dose of 50 cGy was com-
pared with that of a 200 cGy absorbed dose, the difference was statistically significant
(p <0.001, α= 0.05, n = 31). Comparing the optical density growth for a 200 cGy absorbed
dose to a 400 cGy absorbed dose showed a statistically significant difference between the two
datasets (p <0.001, α= 0.05, n = 31). Furthermore, when the optical density growth for a 400
cGy absorbed dose was compared to that of an 800 cGy absorbed dose, a statistically significant
difference was observed (p <0.001, α= 0.05, n = 31). From the results, it was observed that
after 24 hours (1440 minutes) of exposure, the optical densities of all the exposed films were
stabilized.
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Calibration curve for EBT2 film
The calibration curve for the EBT2 film is shown in Fig 4 and was obtained by plotting
absorbed dose measurements obtained from the ionization chamber against the NOD of EBT2
films. Fig 4 shows that as the absorbed dose increased, the NOD of EBT2 films also increased.
A 3rd-degree polynomial represented the line of best fit that relates the net optical density to
absorbed dose and is shown in Eq 12. The correlation coefficient for the plot was estimated to
be 0.9989. The calibration curve was used to convert optical density measurements to the
absorbed dose.
y¼1685X3187:9X2þ1006X2:328 ð12Þ
Where Xrepresents the optical density and y the absorbed dose in gray.
For the same batch number of films and irradiation conditions, the characteristics of the
calibration curve may change depending on the densitometer or scanner used. Hence, the cali-
bration curve should be re-evaluated when a different densitometer or scanner is used for the
optical density measurements.
Fig 3. Post-irradiation optical density growth of EBT2 films. As the time after the films’ irradiation increased, a
corresponding increase in the optical density growth is observed. At longer post-exposure times, the optical density
growth stabilized.
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Fig 4. Calibration curve for converting the film optical density readings to absorbed dose in gray. The absorbed
dose measurement by the ionization chamber was plotted against the NOD of the films.
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Dose uncertainty budget
Fig 5 shows a plot of dose uncertainties against estimated absorbed doses using EBT2 films.
From Fig 5, increasing the absorbed radiation dose increased the absorbed dose uncertainty.
The minimum dose uncertainty recorded was 0.12% (at 50cGy), while the maximum was
1.72% (at 800 cGy).
Assessing the effects of irradiation parameters
Fig 6 shows the effects of irradiation parameters on optical density measurements by the unen-
capsulated and encapsulated film. The results in Fig 6 were normalized under reference condi-
tions (10 cm x10 cm; SSD = 100 cm; Gantry angle 0
◦
). From Fig 6A, there was a general
increase in the normalized optical density (ND) of the unencapsulated film compared to the
encapsulated film when field size increased. The increase in ND was due to the increasing
number of contaminating electrons from the collimator and air with an increase in field size.
Because of the low penetrating power of the contaminating electrons, they were absorbed by
the unencapsulated film. Changes in the field size had a relatively negligible effect on the
encapsulated film because the encapsulation material absorbed most of the contaminating
electrons. The difference in the response of encapsulated and unencapsulated film for varia-
tions in field size was statistically significant (p = 0.0193,α= 0.05,n = 11).
From Fig 6B, as SSD increased, the optical density of the films (both unencapsulated and
encapsulated) decreased due to the decrease in the dose rate of the Equinox 100 Co-60
machine with increasing SSD. At shorter SSD, the films were exposed to large quantities of
low-energy photons scattered by components in the Co-60 teletherapy unit, inducing a slight
over-response of the film. At large SSD, this contamination was insufficient to contribute to
the absorbed dose. For changes in SSD, ND for the encapsulated films was statistically not dif-
ferent from that of the unencapsulated films (p = 0.1802,α= 0.05,n = 11).
Fig 6C shows the effects of gantry angle on the measurements of the unencapsulated and
encapsulated films. From Fig 6C, the response of the encapsulated EBT2 film with gantry
angle was almost constant. However, there was a general increase in the optical density mea-
surements for the unencapsulated film when the gantry angle increased due to the shift of
Fig 5. Plot of total dose uncertainty against absorbed dose for EBT2 film. The percentage uncertainty in absorbed
dose estimations increased as the dose to the film increased.
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contaminating electrons onto the film. Maximum film readings for the unencapsulated film
occurred at gantry angles between 70ºand 80º. A statistically significant difference was
obtained when we compared the ND of the unencapsulated film with the encapsulated film
(p = 0.0018,α= 0.05,n = 24).
From Fig 6D, at a reference field size of 10 cm ×10 cm, an increase in the wedge angle
decreased ND for both the unencapsulated and encapsulated film. The decrease in ND was
because as the wedge angle increases, the thickness of attenuating material increases, causing
an increase in the average energy of the primary and secondary photons. Comparing the ND
of the encapsulated film with the unencapsulated film did not result in a statistically significant
difference (p = 0.6834,α= 0.05,n = 4).
Skin dose assessment
At reference irradiation conditions, we assessed the impact of encapsulation material on skin
dose for a prescribed absorbed dose of 200 cGy to Dmax.
a) Skin dose estimation without encapsulation material
Dskin ¼Df Fcal;skin
Df ¼0:05 48:7104cGy
Fcal;skin ¼1:00
Therefore :Dskin ¼48:7104 1:00 ¼48:71cGy
%skin dose ¼48:71
200:00 100
%Skin dose 24%
Fig 6. Effects of irradiation parameters on the optical density measurements of unencapsulated and encapsulated
film. The results in Fig 6 were normalized to reference conditions (10 cm x10 cm; SSD = 100 cm; Gantry angle 0
◦
). A:
Shows the effect of field size, B: Shows the effect of SSD, C: Shows the effect of gantry angle, D: Shows the effect of
wedge angle.
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b) Skin dose estimation with encapsulation material
Dskin ¼Df Fcal;skin
Df ¼0:14 140:8152cGy
Fcal;skin ¼1:00
Therefore :Dskin ¼140:08125 1:00 ¼140:08cGy
%skin dose ¼140:08
200:00 100
%Skin dose 70%
From the preliminary percentage skin dose estimation for the unencapsulated and encapsu-
lated film, the encapsulation around the film increased the skin dose by approximately three
times under reference conditions. Only the unencapsulated films were used for the subsequent
skin dose measurements due to the high skin dose associated with using encapsulated films.
In Fig 7, the effects of irradiation parameters on skin dose depositions are assessed under
reference irradiation conditions. Increasing the field size increased the skin dose due to the
increase in contaminating electrons. As SSD increased, the deposited skin dose decreased
because only electrons or photons with sufficient energy interacted with the skin. Large gantry
angles had relatively high values in skin dose due to the shift of the contaminating electron
region towards the skin. Furthermore, increasing the wedge angles decreased the skin dose
due to increased attenuation material volume. The skin dose observed with wedged fields was
relatively lower than those of open irradiation fields. For example, at reference conditions of
field size, SSD, and gantry angle, using a 45ºwedge filter reduced the skin dose by a factor of
two compared to an open field.
Entrance dose assessment
Fig 8 shows the percentage accuracy in entrance dose estimation for variations in field size,
SSD, gantry angle and wedge angle. The percentage accuracy in entrance dose measurements
was calculated by comparing the estimated entrance dose using the film with the prescribed
dose. As shown in Fig 8A, for field size ranging from 4 cm x 4 cm to 24 cm x 24 cm, the maxi-
mum and minimum percentage entrance dose difference were 2.1% (at field size of 6cm x 6
cm and 12 cm x 12 cm) and 0.3% (at field size of 18 cm x 18 cm). From Fig 8B, variations in
the SSD resulted in a maximum percentage entrance dose of 2.3% at SSD of 90 cm and a mini-
mum percentage entrance dose of 0.4% at SSD of 75 cm. As illustrated in Fig 8C, the maxi-
mum and minimum percentage entrance dose differences were 2.32% (at a gantry angle of
70º) and 0.03% (at a gantry angle of 10º) for gantry angles ranging from 0ºto 90º. Furthermore,
from Fig 8D, for field sizes ranging from 4 cm x 4 cm to 18 cm x 18 cm, when the wedge angle
changed from 15ºto 60º, the maximum percentage entranced dose difference was 1.72% (at a
field size of 4 cm x 4 cm and wedge angle of 45º). The minimum percentage entrance dose dif-
ference was 0.01% at a field size of 12 cm x 12 cm and a wedge angle of 60º.
Discussion
Radiation therapy involves the delivery of a prescribed dose to the target volume, sparing the
surrounding healthy organs and tissues as much as possible. Traditional techniques that ensure
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the prescribed dose gets delivered to the target volume include treatment planning systems
and phantom studies. These dose monitoring methods are done before the radiation treatment
and may not reflect the radiation delivered during treatment. In vivo dosimetry is used during
radiotherapy to assess the real-time absorbed dose to the target volume. There are several in
vivo dosimeters available; however, GafChromic EBT2 film was used in this study due to its
Fig 7. Effects of irradiation parameters on skin dose deposition. The results of the skin dose depositions were obtained for a reference dose of 200cGy A:
Effect of Field size; B: Effect of SSD; C: Effect of Gantry angle; D: Effect of Wedge angle.
https://doi.org/10.1371/journal.pone.0271000.g007
Fig 8. Effects of irradiation parameters on the estimations of entrance dose. The results were obtained with the
same reference dose of 200 cGy. A: Shows the effect of field size, B: Shows the effect of SSD, C: Shows the effect of
gantry angle, D: Shows the effect of wedge angle.
https://doi.org/10.1371/journal.pone.0271000.g008
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tissue-equivalent properties and ease of handling. Dose measurements using EBT2 film are
affected by contaminating electrons generated in the air and collimator head of the treatment
machine. The production of these electrons is influenced by factors such as photon energy,
beam shaper, treatment field and SSD. In this study, a new configuration of the film setup was
introduced such that the film is within an encapsulation material (made of Perspex). This new
film setup minimizes the effects of the contaminating electrons on film dose estimation. How
this configuration affects dose estimations has not yet been investigated. The general aim of
this research was to characterize GafChromic EBT2 film dose measurements using a tissue-
equivalent water phantom for a Theratron
1
Equinox Cobalt-60 Teletherapy Machine.
Post-irradiation optical density growth
Post-irradiation optical density growth (coloration) is the process whereby a film continues to
increase in optical density (or darken) after irradiation has ceased. As shown in Fig 3, exposing
the film to ionization radiation caused an initial increase in the optical density; however, the
change in optical density stabilized with time. The optical density growth for low doses of radi-
ation stabilized at a shorter time than for high doses of radiation. After 24 hours, there were no
changes in the optical density of all the irradiated films. The result of this analysis is consistent
with that of previous studies [51,52]. In a comprehensive analysis of the Gafchromic EBT2
radiochromic film by Andres, Del Castillo [40] using a 6 MV beam from a Varian 21EX
LINAC (Linear Accelerator) equipped with a Millennium 80 leaf MLC and an Epson 10000XL
flatbed scanner, they observed that stabilization of an irradiated film occurs earlier at low
doses. Aland, Kairn [51] evaluated the Gafchromic EBT2 film dosimetry system for radiother-
apy quality assurance. In their study, the films were irradiated to absorbed doses of 100 cGy,
200 cGy and 300 cGy, by a Varian 21iX linear accelerator (Varian Medical Systems, Palo Alto,
USA) with an energy of 6 MV photon beam and the film readings read with a scanner. They
concluded that after 24 hours of irradiation, the film’s reading for the three doses of radiation
stabilized with time.
Dose uncertainty budget
From the results of this study (using Fig 5), the uncertainty in the absorbed dose estimation
was ±1.79%. The uncertainty estimated in this study is within the range of uncertainty results
determined from previous research [43]. Marroquin, Herrera Gonzalez [43] evaluated the
uncertainty associated with the EBT3 film dosimetry system utilizing net optical density. The
EBT3 film’s composition and thickness of the sensitive layer are the same as those of EBT2
films. However, a matte polyester layer was added to the configuration of EBT3 film to prevent
the formation of Newton’s rings. Compared to EBT2 films, the symmetrical design of EBT3
allows the user to eliminate side-orientation dependence. From the analysis of the response of
the radiochromic film (net optical density) and the fitting of the experimental data to a poten-
tial function, Marroquin, Herrera Gonzalez [43] observed an uncertainty of 2.6%, 4.3%, and
4.1% for the red, green, and blue channels, respectively of an Epson Perfection V750 desktop
flatbed scanner. Thermoluminescent dosimeters (TLDs) are common in vivo dosimeters used
in radiotherapy centers [53–55]. Ferguson, Lambert [53] commissioned and calibrated an
automated TLD facility for measuring exit doses in external beam radiotherapy. Under their
calibration conditions, the uncertainty in a single TLD measurement was approximately ±2%.
The comparable uncertainty measure between TLD and EBT2 film (as determined in this
study) suggests that EBT2 can be used in radiotherapy centers to complement the use of TLD
for in vivo dosimetry.
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Assessing the effects of irradiation parameters
From Fig 6A, as the field size increased, there was a general increase in ND of the unencapsu-
lated film compared to the encapsulated film. The result of this analysis (using just the unen-
capsulated film) is consistent with that of previous studies [56,57]. Jong, Wong [57]
characterized a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) detector for in
vivo skin dose measurement during megavoltage radiation therapy and compared the results
with Markus ionization chamber and Gafchromic EBT2 film measurements. They observed
that the relative dose absorbed by the Gafchromic film increased from 0.3 to 1.7 when the field
size ranged from 1 cm x 1 cm to 40 cm x 40 cm.
As SSD increased, the optical density of the unencapsulated and encapsulated films
decreased (Fig 6B). The changes in the optical density of EBT2 with variations in the SSD are
consistent with findings by Jong, Wong [57]. In their study, they assessed the SSD dependence
and dose rate dependence of EBT2 film and observed that as SSD and dose rate increased, the
optical density of the film decreased.
As shown in Fig 6C, the response of the encapsulated film with gantry angle was approxi-
mately constant. However, increasing the gantry angle increased the optical density measure-
ments of the unencapsulated EBT2 film due to the shift of contaminating electrons onto the
film’s surface. Jong, Wong [57] reported an increase in optical density with gantry angle for an
unencapsulated film. Their study showed that when the gantry angle ranged from 0ºto 90º, a
maximum absorbed dose occurred at 70. The increased film’s response was attributed to the
shift of the charged particle region towards the surface of the phantom. This shift had a negligi-
ble effect on the encapsulated film because the encapsulation material absorbed and prevented
the contaminating charged particles from reaching the film within.
From Fig 6D, increasing the wedge angle decreased the optical density for both the unen-
capsulated and encapsulated film. The decrease in optical density with increasing wedge angle
has been reported in previous studies [43,54,55]. Bilge, Ozbek [56] measured the surface dose
and build-up region with different wedge filters for photon beam energies 6 and 18 MV. They
observed that for both 6 and 18 MV photon energy, an increase in the wedge angle decreased
the measured radiation dose. In the presence of wedge filters, the measured doses by the
encapsulated films were generally higher than that of the unencapsulated films due to the
increase in the average energy of the photon beam [56].
Use of the encapsulation material
Gafchromic EBT2 film used in radiotherapy is highly influenced by contaminating electrons at
high field size and gantry angle [49,58–60]. These contaminating electrons can increase the
computational cost, time, and margin of measurement errors during in vivo dosimetric proce-
dures where skin dose estimation is not a primary focus. From Fig 6, the response of the
encapsulated film to variations in irradiation parameters was relatively constant (for field size
and gantry angle) compared to the unencapsulated film. The fixed response of the encapsu-
lated film without contamination at high field size and gantry angles implies high measure-
ment accuracy, low computational cost, and time for procedures where skin dose estimation is
not required. However, one drawback observed with using the encapsulated film for entrance
dose measurement was the increased contribution of absorbed dose to the skin (70%). The
steep rise in skin dose associated with using the encapsulated film resulted from the shift in
Dmax towards the skin surface.
The design, development, and use of an encapsulation material for EBT2 films are not yet
studied. This study offers a preliminary insight into how using an encapsulation material for a
radiochromic film can influence dosimetric outcomes. Encapsulating the EBT2 films were
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adapted from similar practice using "build-up-caps" with TLDs and MOSFETs for in vivo
dosimetry [61,62]. Sulieman, Theodorou [61] assessed the influence of the geometrical charac-
teristics of cylindrical caps made of various materials at various photon fields on the TL signal.
They observed that caps with appropriate thickness minimized electron contamination and
ensured entrance dose measurement at Dmax. Varadhan, Miller [62] examined the feasibility
of using MOSFET with a brass buildup cap for in vivo dose measurements and compared the
entrance dose measured against that predicted by the treatment-planning system (TPS). They
achieved an overall accuracy within ±5% when the measured doses were compared with that
predicted by TPS. From the advantages observed with using buildup caps with TLDs and
MOSFETs, further studies can be carried out on the design of the encapsulation material to
minimize their perturbation of the treatment field and optimize their use for EBT2 film in vivo
dosimetry.
Skin dose assessment
The skin dose is an important parameter considered during external photon beam radiother-
apy. Previous research has determined the skin to be at risk during radiation therapy [63–65].
Epidemiologic studies have also established a relationship between radiotherapy and the
induction of basal-cell carcinoma [66–69]. Accurate determination of the surface dose is a dif-
ficult but important task for ensuring the proper treatment of patients. The percentage skin
dose values estimated in this study (and shown in Fig 7) are within the range of values pre-
dicted in previous studies [70–73]. In an in vivo dosimetric assessments for an open field 10
cm x 10 cm and SSD of 100 cm, surface doses of 30%, 29.1%, 27.8%, 29.3%, and 29.9% were
determined for five telecobalt machines: Equinox-80, Elite-80, Th-780C, Th-780, and Bhabha-
tron-II respectively [70]. Comparatively slightly lower skin doses (~20%) for an open field of
10 cm x 10 cm and SSD of 100 cm have been reported by Thomas and Palmer [72] and Rapley
[71]. From the range of skin dose values predicted in previous studies (from~20% to 30%), the
percentage skin dose estimated in this study (24%) under the same irradiation conditions can
be considered reasonable. Furthermore, Rani, Ayyangar [73] evaluated the skin dose for a
cobalt-60 teletherapy machine and observed that for an open field of 10 cm ×10 cm at 80 cm
SSD, the % skin dose was 31.98%. The percentage of skin dose reported in their study was
slightly higher than that determined in this research (29.33%) under the same irradiation con-
ditions (open field, field size and SSD).
Results from this study further showed that large field sizes and gantry angles contributed
to a substantial deposition of radiation to the skin, similar to results from previous studies [57,
74]. Yadav, Yadav [74] estimated the skin dose deposition by a 6 MV LINAC for open fields
and fields with beam modifiers. Although they used a higher energy photon than what was
used in this study, the characteristics of the skin dose deposition for variations in irradiation
parameters were similar. They observed that relatively high amounts of skin dose were present
at larger field sizes and gantry angles. Their study observed no statistically significant differ-
ence in the skin dose when the SSD ranged from 80 cm to 120 cm. The behaviors of skin dose
with changes in SSD were similar to that observed in this study. Their analysis showed that the
inclusion of beam modifiers during radiotherapy resulted in higher skin dose than open field
irradiations. Jong, Wong [57] characterized the MOSFET detector for in vivo skin dose mea-
surement during megavoltage radiotherapy. As part of their study, they assessed the character-
istics of EBT2 film for varying irradiation parameters and observed a higher dependency of the
film on field size and gantry angle. Jong, Wong [57] also showed that increasing the wedge
angle decreases the skin dose because the presence of a physical wedge hardens the photon
beam by absorbing scattered radiation.
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Entrance dose assessment
In vivo dosimetry during radiotherapy is necessary to assure that the dose delivered to the
patient corresponds to the prescribed dose, as calculated by the treatment planning system.
Entrance dose measurement is one component of in vivo dosimetry and is defined as the dose
delivered at Dmax. The International Commission on Radiation Units and Measurements
(ICRU) recommends target dose uniformity within ±5% of the absorbed dose delivered to a
well-defined prescription point within the target. From Fig 8, for variations in the irradiation
parameters, the measured entrance doses were within 3% of the target prescribed dose to
Dmax. The degree of accuracy of the entrance dose estimations in this research agrees with a
similar study by Arjomandy, Tailor [75]. In the study by Arjomandy, Tailor [75], EBT2 film
was assessed as a depth-dose measurement tool for radiotherapy beams over a wide range of
energies and modalities. They observed that for the Cobalt-60 photon beam, the percentage
depth doses measured with the EBT2 film showed an excellent agreement (within 2%) with
those measured with ionization chambers.
Traditionally, TLDs and diodes have been the choice for use as an in vivo dosimeter due to
their high accuracy in entrance dose determinations [76–81]. Evwierhurhoma, Ibitoye [76]
determined the role of TLDs during in vivo dosimetry. Their study was part of quality control
and audit in conventional radiotherapy procedures delivered with a Co-60 teletherapy
machine. They showed that no significant difference existed between the prescribed dose and
measured dose of the breast with a percentage deviation difference of less than 5%. Gadhi,
Fatmi [81] developed an absorbed dose verification program using the diode in vivo dosimetry
system for entrance dose measurements. Phantom studies carried out in their work showed
that the percentage difference between measured and calculated dose for entrance setting
remained within ±2%. Their analysis also showed that entrance dose estimations for patients’
measurements were within ±5% (of the prescribed dose) for open fields and ±7% for wedged
fields. Comparisons of the percentage dose difference between measured and calculated
entrance dose estimations for the traditional methods of in vivo dosimetry (both TLDs and
diodes) and EBT2 films suggest that these films can be used as a complementary check to
TLDs and diodes in vivo dose measurements.
Limitations
Gafchromic EBT2 film has an asymmetry in the configuration of layers within it. While studies
on the effect of scanning the film on the “front” side versus the “back” side by Desroches, Bou-
chard [82] and Carrasco, Perucha [47] confirmed a significant difference in optical density
measurement. Aldelaijan, Devic [83] did not observe any significant difference in optical den-
sity measurement. In this study, we scanned only the “front” sides of the films; therefore, the
effects of scanning just the “back” side on the findings of this research were not determined.
The in vivo dosimetric protocol implemented in this research considered only the film’s mea-
surement of entrance dose and the skin dose at the entrance of the photon beam. It is possible
to estimate the exit dose and subsequently the skin dose at the exit side of the photon beam;
however, this was outside the scope of the current study. In this research, only the central axis
absorbed doses were considered. Characterization of the film was not performed for peripheral
doses. The uncertainties in the dose measurements (±1.72%) were obtained under phantom
irradiation conditions. In clinical practice, higher dose uncertainties than that determined in
this study may be expected due to patient-related factors such as movement, shape and compo-
sition and the variation and estimation of patient size during treatment.
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Conclusion
In vivo dosimetry enables the measurement of the radiation dose delivered to a target volume
during radiation therapy. A tissue equivalent water phantom was used to simulate the human
environment. We used GafChromic EBT2 film to assess the skin dose and entrance dose. Two
configurations of the film were considered, an unencapsulated film and the other a film encap-
sulated within a Perspex material. From the post-exposure optical density growth, 24 hours
after irradiation was determined to be optimum in the measurement of the optical density of
the films. In this research, the uncertainty in dose measurements (±1.72%) was within the lim-
its of acceptable uncertainty in radiotherapy absorbed dose delivery of ±5% specified by ICRU
[84]. We assessed the film’s responses for varying irradiation conditions of field size, SSD, gan-
try angle and Wedge angle. The unencapsulated film was observed to be more influenced by
the irradiation parameters than the encapsulated film. A statistically significant difference was
observed when the responses of the unencapsulated and encapsulated film were compared for
varying field size (p = 0.1802, α= 0.05, n = 11) and gantry angle (p = 0.0018, α= 0.05, n = 24).
However, no statistical difference was observed when the responses of the unencapsulated and
encapsulated film were compared for changes in the SSD (p = 0.1802, α= 0.05, n = 11) and
wedge angle (p = 0.6834, α= 0.05, n = 4).
Skin dose assessment by the two film configurations at reference conditions showed a very
high radiation dose deposition by the encapsulation material (70%) compared to the EBT2
film without encapsulation (24%). Due to the very high skin dose deposited by the encapsula-
tion material, we did not consider the encapsulated film configuration in the subsequent skin
dose and entrance dose quantification. The skin dose measured using the film was higher in
open field configurations compared to wedged field configurations. The estimation of the
entrance dose using the unencapsulated film was within 3% of the prescribed absorbed dose
for the variation in irradiation parameters considered in this study.
Supporting information
S1 Dataset.
(PDF)
Author Contributions
Conceptualization: Daniel Akwei Addo.
Formal analysis: Daniel Akwei Addo.
Methodology: Samuel Nii Tagoe.
Resources: Samuel Nii Tagoe.
Writing – review & editing: Elsie Effah Kaufmann, Augustine Kwame Kyere.
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