Introduction
Brachytherapy is classified into three types in accordance
with dose rate from the radioactive source in
use. The dose rate of the source is expected to be >12
Gy/h in HDR brachytherapy where Ir-192 isotope is
encapsulated into a cylindrical shape in after-loading
systems.[,,] The clinical application of HDR brachytherapy involves obtaining dose distribution
from the calculation algorithm of a commercial treatment
planning system (TPS). Nowadays, the majority
of HDR brachytherapy TPSs are based on the American
Association of Physicists in Medicine (AAPM)?
Task Group report 43 (TG-43) dose calculation formalism,
whereas previously, a classical calculation
was used.[] Classical dose calculation is based on point source approximation and calculates the dose
rate by the use of the source activity, the tissue attenuation
coefficient, and the exposure rate. In this method,
the dose around the elongated source is computed
by Sievert integration procedure as the summation of
the contributions from point source elements. This
formalism does not take into account the properties
of the source, such as encapsulation and internal construction.
On the other hand, TG-43 dosimetry formalism
uses some dosimetry parameters that depend
on source properties to calculate the dose rate.[,]
The AAPM proposes that these dosimetry parameters
for brachytherapy sources, such as air-kerma strength
(SK), dose rate constant (∧), geometry factor G(r,Π),
radial dose function g(r), and two-dimensional (2D)
anisotropy factor F(r,Π), could be measured experimentally
or calculated through Monte Carlo simulation
(MCS).[] MCS is a reliable and well-known
calculation method for calculation of the brachytherapy
dosimetry parameters since the source could
be modeled realistically. There are studies about the
derivation of the TG-43 dosimetry parameters with
different MC codes in the literature.[,,] The dose
accuracy of a radioactive source could be achieved
experimentally by the use of appropriate dosimeters.
However, it is difficult to position the detector with
an acceptable uncertainty due to the high-dose gradient
at close distances around the source.[,] In
addition, some properties of the dosimeters, such as
linearity, material composition, energy dependence,
and spatial resolution, should be taken into account
with regard to dose accuracy as well. Thermoluminescent
dosimeters and Gafchromic films are the most
common dosimeters in brachytherapy owing to their
distinct properties.[]
HDR has become the most commonly used treatment
modality in clinical brachytherapy due to its
advantages, such as rapid dose reduction, less doses
to the surrounding anatomical structures, and quick
implementation and personal radiation safety. Dose
distribution in HDR brachytherapy is calculated by
commercial proprietary TPS software packages that
include complicated algorithms to perform the evaluation
because of the presence of hot spots and highdose
gradients at close distances to the source. The
source moves in steps through the applicator to attain
an intended irradiation length in HDR brachytherapy.
The number of dwell positions, the source step size,
and the dwell times can be adjusted according to the
target volume as part of the TPS optimization procedure.[,] The prescribed dose is delivered in several fractions to the patient, and accordingly, the dose
amount of each treatment is relatively high. Therefore,
the dosimetric accuracy of TPS in HDR brachytherapy
is quite essential. There are many studies that were
conducted with MCS in the literature related to the
verification of dose distribution generated by different
TPSs around various sources and applicators.[-]
Naseri et al. performed dosimetric verification of an
HDR TPS, based on the Sievert integration method for
dose calculation, using MCNP4C code for three different
Co-60 sources.[] Toossi et al. performed the dosimetric
validation of an HDR TPS, which uses Sievert
integration for dose calculation, for irradiation lengths
of 8 and 10 cm by the movement of the Co-60 source
along the treatment axis.[] Hadad et al. performed
dosimetric verification of a TG-43 based on TPS for a
nasopharynx case by means of MCS.[16] To the best of
our knowledge, there is no study regarding dosimetric
validation of an HDR TPS, which is based on TG-43
formalism, for various irradiation lengths using MCS
in the literature. The aim of the present study was to
calculate the transverse dose rate profiles of various irradiation
lengths in the range of 2 cm?5 cm, which
are the most commonly used source lengths clinically,
with MCS and to compare the simulation results with
those calculated by Nucletron Oncentra TPS. Accordingly,
it can be possible to investigate the effect of the
irradiation length on the dose difference between
MCS and TPS.
Methods
Nucletron Oncentra Treatment Planning System
The calculation algorithm of Nucletron (Nucletron;
Elekta Company, Elekta AB, Stockholm, Sweden) Oncentra
MasterPlan TPS (version 3.2) is based on the
AAPM TG-43 formalism that performs the dose rate
calculations in a water medium with fixed size for radiation
scattering and makes an assumption of the encapsulated
brachytherapy source as cylindrically symmetric.
This particular TPS has the concept of virtual
dose rate to generate the isodose contours around
the irradiation volume since the intended dose distribution
is created by the stepwise movement of the
source. When the source moves to the next step, the
dose rate of any arbitrary point around a dwell position
decreases. Therefore, the summation of the dose
from each dwell position to any point of interests generates
the virtual dose rate at that specific point. Each
dwell position has a weight ranging between 0.0 and
1.0 in Oncentra TPS that can be changed according to the clinical goal. In the present study, four different
irradiation lengths of 2, 3, 4, and 5 cm were created in
Oncentra TPS. The step size of the source was set to
0.25 mm; therefore, the numbers of dwell positions
were 9, 13, 17, and 21 for irradiations of 2, 3, 4, and 5
cm, respectively.
The activity of the Ir-192 source was 7.965 Curie,
and the irradiation times were generated through the
calculation algorithm of TPS based on source activity
and dose normalization. All dwell positions were
weighted equally, and a dose of 10 Gy was normalized
to 1 cm away from the source for each irradiation scenario.
Therefore, the total dwell time of each case was
different as various numbers of dwell positions were
contained within.
Absolute dose rate profiles were calculated in the
transverse directions from the center of the source at
a distance of ±10 cm for every irradiation length. The
calculated dose values by TPS were normalized to the
air-kerma strength of the source in use stated in the
certificate of the vendors for comparing them with the
simulation results.
MCNP Transport Code
Monte Carlo N-Particle (MCNP) Transport Code is
a MCS code used for computation of radiation interactions
including photons, neutrons, and electrons in
any desired combinations. In this method, individual
radiation particle"s characteristics, such as energy, position,
flight direction, track length, and the energy fluence
on a plane or the dose deposition in a voxel, could
be calculated by the use of the random number engine.
[17] This simulation code was used in the study for
modeling the Ir-192 source and calculation of the dose
around the mentioned irradiation lengths.
In the present study, Nucletron HDR 192Ir Microselectron
v2 (model mHDR-v2) is the source remote
controlled by the after-loading device. All components
of the source with geometrical properties and
material compounds were realistically modeled with
MCNP. Nucletron mHDR-v2 source consists of a central
cylindrical Iridium core (density: 22.42 g/cc, diameter:
0.65 mm, and height: 3.6 mm) and a surrounding
outer capsule (AISI-316; outer diameter: 0.9 mm and
thickness: 0.125 mm) made of stainless steel (density:
7.92 g/cc and weight fraction: 1% Si, 17% Cr, 2% Mn,
65.5% Fe, 12% Ni, and 2.5% Mo).
The source strength of a radioactive source is specified
by the air-kerma strength (SK) in brachytherapy.
Air-kerma strength is one of the TG-43 dosimetry parameters,
and its quantity varies depending on the content and geometry of the source. Therefore, SK should
be calculated for every source in question. In the literature,
the dose rate around the source is usually specified
by normalizing to the air-kerma strength (SK) in
U (1 U=µGy m2/h).[,,] In the present study, 36
spherical detectors were located at the points aligned
with equal angle intervals of 10° on a circle with a 1
m radius around the source (in upright position) to
compute the air-kerma strength of the involved source.
Air-kerma calculations were performed in vacuum
medium as suggested by the TG-43 report.
The source lies and moves along the y-axis in the
simulations with MCNP as in Oncentra TPS. The irradiation
scenarios of 2, 3, 4, and 5 cm were created in
a water phantom of 40×40×40 cc (density: 1 g/cc and
weight fraction: 11.19% H and 88.81% O) and were
surrounded by spherical dose cells made of water with
a radius of 0.25 cm. Dose cells were positioned along
the central plane of the associated irradiation lengths
to scan an area of 10×10 cm2. An air sphere (density:
0.001205 g/cc, weight fraction: 0.0124% C, 75.5268%
N, 23.1781% O, and 1.2827% Ar, and radius: 50 cm)
was defined around the water phantom. Figure 1 depicts
a plot of the geometry in question as drawn by the
MCNP code. All parameters of the mentioned irradiation
scenarios, such as the coordinates and the number
of dwell positions, the step size of the source, and the
dwell times, in MCNP were set to the same quantities
with those in TPS. The dose in each cell was obtained
as the summation of the doses from each dwell position.
Fig 1: MCNP plot of the source, phantom, and detectors in question. All dimensions are in cm. (a) xz (y = 0) phantom +
transverse view of the source. (b) xy (z = 0) phantom + coronal view of the source. (c) xy (z = 0) phantom + source
+ detectors. (d) xy (z = 0) complete geometry.
Only photons were taken into account with regard
to radiation transport in the simulations without
specifying any flight directions of the source particles
(4? isotropic distribution). The energy spectrum of
the Ir-192 source was defined as containing both the
gamma and the X-ray lines of the isotope. The energy
deposition (MeV/g/particle) tally (F6) was used in the
simulations to record the absolute dose in the point of
interests. The statistical uncertainty was achieved to be
<2% for all sets of simulations as a result of running
100 million photon histories per set.
Results
The mean value of air-kerma strength (SK) was computed
as 9.79×10-8 (9.465×10-8?9.832×10-8) U/Bq using
MCNP simulations. The air-kerma strength per
activity at a point 1 m away along the transverse axis
of the source was calculated as 9.86×10-8 µGy m2/h/Bq.
All absolute doses obtained by MCNP were normal ized
by the last mentioned SK value since it was the SK
value of the TG-43 reference point.
The percentage difference between the calculated
doses by MCNP and TPS at the points of interest was
obtained according to the following equation:
The dose rate profiles per air-kerma strength, calculated
by MCNP and TPS, on the central axis of the irradiation length along the x-axis (perpendicular to the
source movement direction) are shown in Figures 2a,
b, c, and d for irradiation scenarios of 2, 3, 4, and 5 cm
lengths, respectively.
Fig 2: Comparison of absolute dose rate profiles, per U, calculated by MCNP and Oncentra TPS on the central axis of the
irradiation length along the x-axis (perpendicular to the source movement direction). Irradiation lengths of (a) 2
cm, (b) 3 cm, (c) 4 cm, and (d) 5 cm.
Dose rates per U calculated by TPS were found to
be 2.82%, 2.82%, 2.80%, and 2.73% lower than those computed by MCNP at close distances (x=2 mm) to
the source center for 2, 3, 4, and 5 cm irradiation scenarios,
respectively. The difference between MCNP
and TPS calculated dose rates is <1% at distances
farther than 6 mm away from the source center (x≥6
mm) and >1% beyond 5 cm from the irradiation center. The differences between TPS and MCNP were
calculated as 2.51%, 3.31%, 3.44%, and 3.35% at 8 cm
and 4.03%, 4.68%, 4.82%, and 5.21% at 10 cm away
from the source center for irradiation lengths of 2,
3, 4, and 5 cm, respectively. The dose differences between MCNP and TPS have been observed to increase
with irradiation length at distances farther than 6 cm
from the center. However, the dose differences for all
irradiation scenarios are almost equal to each other at
the points near the source.
Discussion
It is quite important to accomplish the accuracy of dose
distribution as calculated by TPS in high-dose rate
brachytherapy due to the presence of a steep dose gradient
around the irradiated site and the considerable
high dose per fraction. MC can be used for calculation
of TG-43 dosimetry parameters and doses around the
radioactive source by taking into account the radiation
interactions for involved source and medium. In
the present study, dosimetric validation of Nucletron
Oncentra TPS was performed with MCNP simulation
code for irradiation scenarios of 2, 3, 4, and 5 cm
source lengths.
There have been published studies regarding MC
calculation of TG-43 dosimetry parameters of Nucletron
mHDR-v2 Ir-192 source. Granero et al. modeled
Nucletron mHDR-v2 and mHDR-v2r (new design)
Ir-192 sources with three different MC codes, namely
MCNP5, PENELOPE2008, and GEANT4, to calculate
and compare the TG-43 dosimetry parameters that are
dose rate constant, radial dose function, and 2D anisotropy
factor.[] They did not observe any noticeable
difference between the dosimetry parameters of
the above-mentioned Nucletron Ir-192 source beyond
0.25 cm distance (r?0.25 cm) from the source center.
In another published study, air-kerma rate and other
TG-43 dosimetry parameters were calculated for the
mentioned sources in the previous study with PENELOPE2008.[] Lopez et al. calculated the air-kerma
rate of Nucletron Microselectron mHDR-v2 and
mHDR-v2r to be 9.78×10-8 U/Bq and 9.86×10-8 U/
Bq, respectively, at the point 1 m away from the source
on the perpendicular direction to the axis where the
source lies (θ=Π/2). In our study, the air-kerma rate of
Nucletron mHDR-v2 Ir-192 source was computed as
9.86×10-8 U/Bq at the same point of their study. There
is a good consistency between our study and their study
as our calculated air-kerma rate differs 0.8% from the
value calculated by Lopez et al.[]
MC technique has been used frequently for validation
of various TPSs in addition to the calculation
of TG-43 dosimetry parameters of different HDR
brachytherapy sources. Naseri et al. calculated dose
distribution around three different Co-60 sources with
MCNP4C code. Thereafter, the simulated results were
compared with those calculated by a classical calculation
based on an HDR TPS. They found a large deviation
between MC and TPS calculated isodose lines
near the source region.[] They attributed this difference
to the high-dose gradient at close region around the HDR source. Hadad et al. studied the HDR treatment
plan of a patient with nasopharynx cancer by
using computed tomography (CT) images of a patient
and a water equivalent phantom with DOSXYZnrc
software. The dose distribution around the source in
the treatment plan was calculated in water equivalent
and virtual phantom obtained out of the patient CT
images separately. The simulated values in water equivalent
phantom were compared with the dose distribution
calculated by Oncentra TPS, which is based on the
TG-43 formalism for the same irradiation parameters.
In their study, two isodose values of 0.25 Gy and 1 Gy
were calculated with MC and TPS. They concluded
that Oncentra TPS calculates lower dose values at the
near source region and higher ones at further distances
from the source than MC.[] The dose rate distribution
of the new design Microselectron Nucletron HDR
Ir-192 source in the near source region has also been
studied, and dose rate has been observed to be affected
by the non-equilibrium of the charged particle at
close distances (0.5≤r≤2 mm) to the source.[] In our
study, TPS is also observed to calculate lower values
within the difference of 2.82% than MC at 2 mm distance
from the source. This deviation decreases as the
distance from the source increases. However, beyond
8 mm of the source, the TPS calculated values become
0.5% larger than MC for all irradiation scenarios. Our
results are in good agreement with literature findings.
In the present study, TPS calculated doses are lower beyond
3.5 cm distance from the source. We found larger
discrepancies between MC and TPS beyond 10 cm
distance for all the irradiation lengths studied. In the
present study, TPS calculates the dose according to the
TG-43 report. In this formalism, radial dose function
is an important dosimetry parameter that indicates the
dose variation by means of photon interactions, such
as absorption and scattering in transverse plane. HDR
TPSs use the fifth-order polynomial fit for tabulated
g(r) data.[] Chandola et al. calculated the radial dose
function of the Nucletron mHDR-v2 Ir-192 source using
EGSnrc and Geant4 codes and measured with the
in-air ion chamber.[] They compared their findings
with each other and with the findings by Daskalov.[]
They found good agreement between the MC calculated
and measured g(r) values up to 5 cm distance from
the source center. However, the deviation between the
obtained g(r) values was observed to slightly increase
at further distances from the source. This increased deviation
was ascribed to the volume average effect of the
ion chamber and various other uncertainties. The deviation
between the simulated g(r) values with different software packages increases as the distance increases
beyond 8 cm from the source from their tabulated and
drawn radial dose function. We also observed the larger
discrepancies between the MC and TPS calculated dose
rate values on the transverse plane of the irradiation
length. The observed largest deviation at 10 cm away
from the source increases as the dwell positions or irradiation
length increases. This deviation increase might
be attributed to the variation of the radial dose function
at large distances from the source.
Conclusion
Dose rate profiles calculated by Oncentra HDR
brachytherapy TPS have been verified with MCNP
software for four different irradiation lengths between
2 and 5 cm. There is a good consistency between TPS
and MCNP calculated dose rate profiles at distances
between 2 mm and 8 cm away from the source. The
dose difference is obtained within 2% at above the
mentioned distance interval for all the irradiation
scenarios. The larger dose difference of approximately
2.8% was observed at the near source region, and
this deviation was found to be independent from the
number of dwell positions. The discrepancy between
TPS and MC calculated dose rate values increases as
the distance from the source increases beyond 5 cm
from the source center, and its amount has been affected
by the irradiation length or in another word
number of dwell positions. The largest difference of
5.2% has been found at 10 cm distance away from the
source for 5 cm irradiation length.
Peer-review: Externally peer-reviewed.
Conflict of Interest: None declared.
Ethics Committee Approval: No ethical approval was required
for this study.
Financial Support: None declared.
Authorship contributions: Concept - G.K.; Design -
H.A.B., A.B., G.K.; Supervision - G.K., A.B.; Materials -
H.A.B., A.B., G.K.; Data collection &/or processing - H.A.B.,
A.B., G.K.; Analysis and/or interpretation - H.A.B., A.B.,
G.K.; Literature search - H.A.B.; Writing - H.A.B.; Critical
review - G.K., A.B.
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