Introduction
In the west, endometrial carcinoma is the most common
malignancy among women; however, in India,
the incidence rates are low.[1] Most endometrial cancers are confined to the uterus.[2] If the malignancy
is confined to the uterus, then the standard treatment
is abdominal hysterectomy and bilateral salpingo-oophorectomy.
In most cases, the surgery is followed by
external beam radiotherapy to the pelvis to reduce the chances of recurrence. In some cases, depending on the
tumor grade and depth of myometrial invasion, external
beam radiotherapy is then followed by intracavitary
brachytherapy. A study was conducted by the Gynecologic
Oncologic Group in patients with surgical Stage
Ib and Ic (FIGO 2018) disease and negative lymph node
metastases, wherein it was found that although the adjuvant
pelvic radiation reduced recurrences; however,
there was no significant difference in survival.[3] The
main advantage of pelvic radiotherapy is the reduction
in vaginal recurrences, so vaginal brachytherapy alone
can be justified in patients with no pelvic node metastasis
because it has fewer complications.[4] The ASTEC
study and PORTEC 2 study reports resulted in the use
of brachytherapy alone in the majority of intermediaterisk
patients.[5-11] Patients who receive brachytherapy
alone may have a better quality of life, as it results in
fewer gastrointestinal complications.[12-14]
Intracavitary high dose rate (HDR) brachytherapy
of the vagina can be given by various types of applicators,
and the segmented vaginal cylinder is one of them.
These vaginal applicators are made up of comparatively
high Z material and have a high density compared to
water. In HDR brachytherapy of the vagina, the applicator
is first inserted into the patient, and then the patient
is taken for CT imaging. Finally, the same CT scan
is transferred to the treatment planning system (TPS).
A suitable patient plan is then created with the help
of the TPS. The Task Group No. 43 (TG-43) formalism
given by the American Association of Physicists
in Medicine is primarily used in most commercially
available TPSs for dose estimations in brachytherapy.
The TG-43 formalism considers a bare source in the
center of water for calculations and does not account for heterogeneity correction within the human body.
[15-17] Thus, in brachytherapy TPS, scatter conditions
and photoelectric effect cross-sections in relation to
water are taken into account when calculating the dose,
surrounding the radioactive source. However, in practice,
the segmented vaginal applicators used in vaginal
brachytherapy have a high CT number compared to
water, and as a result, their scattering and absorption
within the human body differ greatly from water, and
thus the attenuation offered by these applicators differs
considerably when compared to water.
The vaginal applicator set studied in this article
consists of vaginal cylinders with varying diameters, as
shown in Figure 1. The universal segmented cylinder
vaginal applicator set can be used for cancer treatment
of the vagina, vaginal stump, and rectum. These are CT
compatible applicators manufactured by Varian and
are compatible with afterloaders such as the VariSource
iX, VariSource 200, GammaMedplus iX, and GammaMedplusTM
having catalog number GM11011100.
Although various types of vaginal applicators are used
for the treatment of endometrial carcinoma, the dose
attenuation due to this type of applicator material has
not been yet studied. This research aims to investigate
the effects of vaginal applicator wall attenuation and
compare it with soft-tissue attenuation. In this study,
the attenuation coefficient of segmented vaginal cylinders
of different diameters was practically measured
using EBT3 Gafchromic film. In addition, a novel technique,
the use of a well-type ionization chamber, was
employed to measure the attenuation caused by different
diameters of segmented vaginal applicators, and the
results were compared with those obtained using Gafchromic
films. The majority of other investigators either used 0.125 cc ion chambers or performed film and
TLD dosimetry for such type of measurements. However,
using a well-type ionization chamber for such
measurements is reasonably easy and quite accurate.
Fig. 1. The Vaginal applicator set. (a) Without being assembled. (b) After being assembled.
Methods
This study was carried out in a tertiary care hospital. The
hospital is equipped with the latest radiotherapy equipment
capable of delivering a precise dose to the target.
The HDR brachytherapy unit used for this study is a
fifth generation, 24 channels afterloader, Varian made,
GammaMedplus IX. The GammaMedplus IX afterloader
is fully integrated with the BrachyVision? TPS. The
dose calculation algorithm in BrachyVision is based on
the TG-43 formalism given by the American Association
of Physicists in Medicine. The GammaMed plus IX
afterloader HDR unit is housed with an Ir-192 radioactive
source. The Ir-192 source housed in the unit has a 3.5 mm active length and 0.7 mm diameter.[-] The
unit is also provided with various types of applicators
for brachytherapy treatment. The Universal Segmented
Cylinder Applicator studied in this article, as shown
in Figure 1, consists of a rigid guide tube of 250 mm
length, front end segment of 20 mm diameter and 50
mm length, front end segment of 25 mm diameter and
52.5 mm length, front end segment of 35 mm diameter
and 57.5 mm length. In addition, the applicator set also
consists of four types of cylinder segments, each having
a length of 30 mm and diameters of 20 mm, 25 mm, 30
mm and 35 mm. The front end segment and the rigid
guide tube allow for a distal first dwell position to enable
a high dose delivery to the vaginal stump. The front end
segment and the cylinder segments are made up of polyetheretherketone
(PEEK) of density 1.32 g/cc, whereas
the rigid tube is made up of polytetrafluoroethylene.
For the intracavitary brachytherapy treatment of endometrial
cancers, the segmented vaginal applicator of appropriate diameter was first inserted into the vaginal
vault of the patient, and then the patient had to undergo
computed tomography (CT) scan. The CT data set was
then transferred to the Brachyvision TPS. The target and
other organs at risk were contoured by the radiation oncologist,
and then the medical physicist performed the
dose optimization process on the TPS so that the prescribed
dose of 7 Gy was received at 5 mm from the surface
of the segmented applicator, as shown in Figure 2. If the plan was deemed satisfactory, the radiation oncologist
would then approve the plan and was transferred to
the control console for the treatment execution.
Fig. 2. Representative dose in color wash for 7 Gy prescription at 0.5 cm from applicator surface in multiplanar view and
three-dimensional view.
Use of EBT3 Gafchromic Films for Dose Measurement
This specific patient treatment plan was delivered on a
phantom, mimicking the patient setup, with EBT3 Gafchromic
film sheets, 1×1 cm2 in size, wrapped in thin,
transparent polythene, and attached to the vaginal cylinder's
surface, as shown in Figure 3a. The films were
adhered with tape on the surface of the vaginal applicator
in such a way that there was no air gap between
the curved applicator surface and the films. The films
were positioned on the vaginal cylinder's surface at predefined
intervals from the distal end of the cylinder so
that the same can be easily reproduced on the TPS. To
imitate backscatter within the patient, the applicator
was then covered with an Elasto-Gel bolus (water-based
gels containing acrylic polymer) with a density of 1.02
g/cm3 and a thickness of 6 cm as shown in Figure 3b.
Fig. 3. (a) Applicator surface taped with EBT3 Gafchromic
films. (b) Applicator surface wrapped in
Elasto-Gel bolus.
For accurate film dosimetry, a standard protocol
was followed.[] The EBT3 films were first exposed
to known doses and then scanned with an Epson
scanner, and the input data thus obtained was fed to
Omnipro software. The same Epson scanner and Omnipro
software were then used to measure the dose
recorded on the exposed EBT3 film placed on the surface
of the vaginal cylinder.
Use of a Well-type Ionization Chamber for Dose
Measurement
The well-type ionization chamber is widely used in
brachytherapy for dosimetric purposes. Well-type ionization
chambers are equipped with a source holder
that is unique to each brachytherapy source and is
commonly referred to as a source jig. The charge collected
by the chamber for a particular brachytherapy
source depends on the source jig used for the measurement.
For Ir-192, the source jig is made up of an
air equivalent material with CT number ranging from
-930 to -980. To obtain the attenuation due to the
various cervix applicators, the source jig was replaced
with water equivalent Elasto-Gel. To mimic the various
vaginal cylinder sizes, the Elasto-Gel of respective diameters
were designed, that is, 35 mm, 30 mm, 25 mm,
and 20 mm as shown in Figure 4. The measurements
were made first with vaginal cylinder within the well
of the chamber and then vaginal cylinder was replaced
with the Elasto-Gel of respective diameter.
Fig. 4. Water equivalent elasto gel of various diameters.
Case 1
The plan created for the treatment of the patients was executed
on HDR 1000 plus well-type ionization chamber,
and the same diameter of the vaginal cylinder was used
in the well of the chamber as shown in Figure 5a. The total
charge collection by the electrometer was noted down.
Case 2
In the second case again the same plan was again executed
on HDR 1000 plus well-type ionization chamber
but in this case, the Elasto-Gel of the same diameter
was used instead of the vaginal cylinder, as shown in
Figure 5b. Again the charge collection by the electrometer
was noted down.
Fig. 5. Setup for the measurement of charge collection
using (a) 35mm Elasto-Gel within the well of
chamber. (b) 35mm vaginal applicator within
well of chamber.
The ratio of charges obtained using the segmented
vaginal cylinder and Elasto-Gel source jig in the welltype
ionization chamber gave the effective attenuation
by the segmented vaginal cylinder, and the same principle
was used in this study to obtain the attenuation
due to various segmented vaginal cylinder applicators.
Pertinently, in both of the aforementioned cases, it
was ensured that the distal end of the vaginal applicator
and Elasto-Gel was placed at the same location in
the base of the well-type ionization chamber. Thus, the
dwell positions of the Ir-192 source within the chamber
were precisely the same in both cases and there was no
difference between the two scenarios in terms of the Ir-
192 source's relative dwell positions inside the chamber.
A similar procedure was repeated for different patients
wherein the patient was first inserted with the appropriate
applicator and then the plan was created on the TPS, and the same plan was executed with Gafchromic
film taped on the surface of the applicator. Subsequently,
the charge collection using a well-type ionization
chamber was also measured. In this way, the results
were obtained for different diameters of applicators.
The TPS calculated dose and the measured dose recorded
by the Gafchromic films were compared using
a paired t-test and examined in the data editor of SPSS
Version 20. Using the same test, the charge collected
using vaginal cylinder and Elasto-Gel in well-type ionization
chamber was also compared. p=0.10 or less was
considered significant.
Ethical statement
The study was approved by the Ethics Review Committee
of Sher I Kashmir institute of Medical Sciences
(reference: SIMS 131/IEC-SKIMS/2022-196). Consent
from patients was not required as datasets were not for
clinical use of patients.
Results
A total of 20 intracavitary brachytherapy insertions were
studied, five insertions for each 35 mm, 30 mm, 25 mm,
and 20 mm diameter vaginal applicator. The dosimetric
results were obtained using both Gafchromic films and a
well-type ionization chamber. The mean overestimation
of dose by the TPS was calculated for each applicator using Gafchromic films. The maximum overestimation
of TPS dose for the 35 mm vaginal applicator was 4.74%
and the least was for the 20 mm vaginal applicator; the
results are shown in Table 1. The graphical representation
of the TPS overestimated dose versus the diameter
of the applicator is shown in Figure 6.
Table 1 Comparison of mean in vitro doses and calculated (treatment planning system ? TPS) doses in intracavitary brachytherapy
(ICBT) with the cylinder applicator
Fig. 6. Graphical representation of percentage overestimation
of dose with applicator size (measured by
using Gafchromic films).
The mean attenuation by the various applicators was
also calculated by using a well-type ionization chamber,
and the results obtained for attenuation by vaginal cylinders were in accordance with the results obtained
using Gafchromic films and are given in Table 2. From
Table 2, it can be observed that the vaginal cylinder of
35 mm resulted in maximum attenuation of 4.456%.
The graphical representation of mean attenuation by
the various applicators versus the diameter of the applicator
is shown in Figure 7.
Table 2 Comparison of mean charge collected for Elasto Gel and mean charge collected with vaginal applicators in a welltype
ionization chamber
Fig. 7. Graphical representation of percentage attenuation
by applicators with applicator size (measured
in well type ionization chamber).
The results of the statistical analysis in both
Gafchromic measurements and well-type measurements showed that there was no significant difference
in the measured doses for different diameters of vaginal
applicators (p>0.10).
Discussion
The attenuation provided by PEEK segmented vaginal
cylinders has been explored in this study, which was
yet to be investigated. Using EBT3 Gafchromic films
and a well-type ionization chamber, the dosimetric
influence of various segmented vaginal cylinders in
brachytherapy dose was determined.
The dose calculation method in BrachyVision TPS
(version 11) is based on the TG-43 algorithm, which effectively
neglects the material of the applicator inserted within the patient and calculates the dose by considering
a radioactive source inside the homogeneous water
medium. There is a difference between the results obtained
from the TPS and practical dose measurements,
and the difference is due to the TPS calculation method,
in which a uniform water equivalent phantom is assumed.
The segmented vaginal cylinders are made up
of comparatively high Z material and have CT number
ranging from 250?300 HU; however, the CT number
of water is zero HU.[] The CT number of material
directly reflects the linear attenuation coefficient of the
material of interest. A higher linear attenuation coefficient
means more interactions per cm and hence more
attenuation. Thus, these segmented cylinders have
quite different dosimetric properties compared to water.
This is further confirmed by the study conducted by Meigooni et al.,[] wherein the authors concluded
that solid water is equivalent to water for dosimetry in
brachytherapy; however, polystyrene and PMMA are
not equivalent to water and are further supported by
the findings of Lewis et al.[]
In this study, the doses at the surface of the various
segmented cylinders were obtained by placing the Gafchromic
films on the surface of the applicator. The effect
of segmented cylinders on net charge collection versus
the charge collection with a water equivalent Elasto-Gel
was also studied using a well-type ionization chamber. As
the charge collection in the well-type ionization chamber
is directly proportional to the dose delivered, thus in both
cases, we effectively measured the effect of segmented
cylinders on the dose delivered to the patient. Both methods
used in this study showed that the brachytherapy
TPS based on TG-43 formalism overestimates the dose.
The dose recorded on the films was measured using
an Epson scanner with IMRT Omnipro software. The
recorded dose on the films is given in Table 1, which
reveals that there is the maximum overestimation of
the TPS dose for the 35 mm vaginal cylinder, and it is
lowest for the 20 mm cylinder.
The percentage attenuation values obtained by using
a well-type ionization chamber are shown in Table 2,
and here, it can also be observed that a vaginal cylinder
of diameter 35 mm resulted in maximum attenuation
and the least attenuation of the dose was due to a vaginal
cylinder diameter of 20 mm. In both the aforementioned
methods, it was found that the attenuation in the
dose increases with an increase in the diameter of the
vaginal cylinder and can be attributed to the increase
in the applicator wall thickness with an increase in diameter
of the vaginal cylinder. The study conducted by Oyekunle et al.[] and Gholami et al.[] concluded
that there is an overestimation of dose by the TPS due
non-water equivalent material of the applicator and the
same should be considered for the better estimation of
doses to the target and organs at risk. Ye et al.[] in
their study using ion chamber and Monte Carlo simulation
found that if the applicator attenuation is neglected
the dose overestimates along the transverse plane by up
to 3.5% and even worsens the error in oblique angles.
In our study, although there is no statistically significant
difference between the TPS and detector measured
dose, both the methods are in agreement with
each other in measuring the TPS overestimation. Accounting
for the dose attenuation by these vaginal cylinders
is therefore expected to improve the HDR dose
delivery accuracy.
Conclusion
This work assesses the dose variance brought on by
the brachytherapy TPS and advances toward the good
practice of HDR vaginal vault brachytherapy treatment.
Although not statistically significant, the overestimation
of the dosage by the brachytherapy TPS based on
the AAPM TG-43 calculation algorithm can be taken
into account for the precise dose estimation to the target
and organs at risk. Therefore, determining the attenuation
coefficients of vaginal cylindrical applicators
and incorporating them into the brachytherapy TPS can
improve treatment accuracy. Thus, we draw the conclusion
that by taking into account the segmented vaginal
cylinder applicator heterogeneity in the TPSs, the dose
calculations in brachytherapy can be improved.
Peer-review: Externally peer-reviewed.
Conflict of Interest: All authors declared no conflict of interest.
Ethics Committee Approval: The study was approved by
the Sher I Kashmir institute of Medical Sciences Institutional
Ethics Committee (no: SIMS 131/IEC-SKIMS/2022-196,
date: 10/05/2022).
Financial Support: None declared.
Authorship contributions: Concept - A.A.K., M.M.H.;
Design - A.A.K., M.M.H.; Supervision - A.A.K., M.M.H.;
Materials - S.Q.W.; Data collection and/or processing -
A.A.K., A.V., M.M.H.; Data analysis and/or interpretation
- A.V., M.M.H., S.Q.W.; Literature search - A.A.K., A.V.,
M.M.H., S.Q.W.; Writing - A.A.K., A.V., M.M.H., S.Q.W.;
Critical review - A.A.K., A.V., M.M.H., S.Q.W.
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