

ORIGINAL ARTICLE 



Year : 2022  Volume
: 47
 Issue : 3  Page : 270278 

Monte carlo study on dose distributions around ^{192}Ir, ^{169}Yb, and ^{125}I brachytherapy sources using EGSnrcbased egs_brachy usercode
Subhalaxmi Mishra^{1}, Bibekananda Mishra^{2}, T Palani Selvam^{3}, Sudesh Deshpande^{4}, Munir Shabbir Pathan^{1}, Rajesh Kumar^{1}
^{1} Division of Radiological Physics and Advisory, Health Safety and Environment Group, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India ^{2} Division of Radiological Safety, Atomic Energy Regulatory Board, Mumbai, Maharashtra, India ^{3} Division of Radiological Physics and Advisory, Health Safety and Environment Group, Bhabha Atomic Research Centre; Homi Bhabha National Institute, Mumbai, Maharashtra, India ^{4} Department of Radiation Oncology, P. D. Hinduja National Hospital and MRC, Mumbai, Maharashtra, India
Date of Submission  09Mar2022 
Date of Decision  01May2022 
Date of Acceptance  13May2022 
Date of Web Publication  8Nov2022 
Correspondence Address: Dr. Subhalaxmi Mishra Division of Radiological Physics and Advisory, Health Safety and Environment Group, Bhabha Atomic Research Centre, Mumbai  400 094, Maharashtra India
Source of Support: None, Conflict of Interest: None  Check 
DOI: 10.4103/jmp.jmp_16_22
Abstract   
Introduction/: As per the recommendations of the American Association of Physicists in Medicine Task Group 43, Monte Carlo (MC) investigators should reproduce previously published dose distributions whenever new features of the code are explored. The purpose of the present study is to benchmark the TG43 dosimetric parameters calculated using the new MC usercode egs_brachy of EGSnrc code system for three different radionuclides ^{192}Ir, ^{169}Yb, and ^{125}I which represent high, intermediate, and lowenergy sources, respectively. Materials and Methods: Brachytherapy sources investigated in this study are highdose rate (HDR) ^{192}Ir VariSource (Model VS2000), ^{169}Yb HDR (Model 4140), and ^{125}I lowdoserate (LDR) (Model OcuProsta). The TG43 dosimetric parameters such as airkerma strength, S_{k}, dose rate constant, Λ, radial dose function, g(r) and anisotropy function, F(r,θ) and twodimensional (2D) absorbed dose rate data (alongaway table) are calculated in a cylindrical water phantom of mass density 0.998 g/cm^{3} using the MC code egs_brachy. Dimensions of phantom considered for ^{192}Ir VS2000 and ^{169}Yb sources are 80 cm diameter ×80 cm height, whereas for ^{125}I OcuProsta source, 30 cm diameter ×30 cm height cylindrical water phantom is considered for MC calculations. Results: The dosimetric parameters calculated using egs_brachy are compared against the values published in the literature. The calculated values of dose rate constants from this study agree with the published values within statistical uncertainties for all investigated sources. Good agreement is found between the egs_brachy calculated radial dose functions, g(r), anisotropy functions, and 2D dose rate data with the published values (within 2%) for the same phantom dimensions. For ^{192}Ir VS2000 source, difference of about 28% is observed in g(r) value at 18 cm from the source which is due to differences in the phantom dimensions. Conclusion: The study validates TG43 dose parameters calculated using egs_brachy for ^{192}Ir, ^{169}Yb, and ^{125}I brachytherapy sources with the values published in the literature.
Keywords: Brachytherapy, egs_brachy, EGSnrc code system, Monte Carlo, TG43 dosimetry
How to cite this article: Mishra S, Mishra B, Selvam T P, Deshpande S, Pathan MS, Kumar R. Monte carlo study on dose distributions around ^{192}Ir, ^{169}Yb, and ^{125}I brachytherapy sources using EGSnrcbased egs_brachy usercode. J Med Phys 2022;47:2708 
How to cite this URL: Mishra S, Mishra B, Selvam T P, Deshpande S, Pathan MS, Kumar R. Monte carlo study on dose distributions around ^{192}Ir, ^{169}Yb, and ^{125}I brachytherapy sources using EGSnrcbased egs_brachy usercode. J Med Phys [serial online] 2022 [cited 2022 Nov 29];47:2708. Available from: https://www.jmp.org.in/text.asp?2022/47/3/270/360590 
Introduction   
As per the American Association of Physicists in Medicine (AAPM) Task Group 43 (TG43) recommendations, Monte Carlo (MC) investigators should reproduce previously published dose distributions for at least one widely used brachytherapy source model whenever new features of the code are explored.^{[1],[2]} egs_brachy^{[3],[4]} is a new usercode of EGSnrc code system^{[5]} designed especially for brachytherapy applications. To the best of our knowledge, TG43 dosimetry parameters are investigated for two highdose rate (HDR) sources ^{192}Ir MicroSelectron V2 and BEBIG ^{60}Co (model Co0.A86) using egs_brachy code.^{[3],[6]} Recently, TG43 parameters are calculated by Safigholi et al.^{[7]} for lowenergy (≤50 keV) photonemitting lowdose rate (LDR) brachytherapy sources (40 numbers) using egs_brachy to update the Carleton Laboratory for Radiotherapy Physics TG43 dosimetry database. TG43 parameters vary significantly with different source designs and encapsulation materials due to the existence of highdose gradient region around it. Hence, it is important to benchmark the dosimetry dataset of a given brachytherapy source model before carrying out further studies using a new MC code.
The purpose of the present study is to benchmark the TG43 dosimetric parameters calculated using the new usercode egs_brachy^{[3],[4]} for three different radionuclides ^{192}Ir, ^{169}Yb, and ^{125}I which represent high, intermediate, and lowenergy sources, respectively. The brachytherapy sources for which TG43 parameters are not available using egs_brachy^{[3],[4]} MC code are chosen for benchmarking and the sources considered for the investigation are HDR ^{192}Ir (Model VS2000),^{[8]} HDR ^{169}Yb (Model 4140)^{[9]} and LDR ^{125}I (Model OcuProsta).^{[10]} Thus, this study covers a range of photon energies relevant in brachytherapy.
^{192}Ir HDR VS2000^{[8]} source is widely in use for clinical applications and differs significantly from other commercially available ^{192}Ir HDR brachytherapy sources in their dimensions such as active length, active diameter, and the encapsulation materials. It consists of two active sources of 2.5 mm each, as compared to the single source of typical active length of about 3.5 mm; the active diameter of VS2000 sources is 0.35 mm as compared to the typical active diameter of 0.6 mm. Alloy of Nickel and Titanium is used as the encapsulation material in VS2000 whereas stainless steel is used in other brachytherapy sources. For VS2000^{[8]} source, Angelopoulos et al.^{[8]} calculated TG43 dosimetric parameters in a 30 cm diameter spherical water phantom using an egs_brachy analytical MC code.^{[11],[12],[13]} In another study, Taylors and Rogers^{[14]} calculated the TG43 dosimetric parameters in a rectilinear water phantom of dimensions of 80 cm × 80 cm × 80 cm for ^{192}Ir VS2000^{[8]} and ^{169}Yb 4140^{[9]} sources using the MC code BrachyDose.^{[15],[16]} Medich et al.^{[9]} calculated TG43 dosimetric parameters in a 40 cm diameter spherical water phantom for ^{169}Yb (model 4140) source using MCNP5 MC code.^{[17]}
OcuProsta is an indigenous model of ^{125}I brachytherapy source designed and fabricated by Radiopharmaceuticals Division of Bhabha Atomic Research Centre for brachytherapy applications.^{[10],[18],[19],[20]} This source is clinically used in permanent prostate implant.^{[20]} It consists of 0.5 mm diameter and 3.0 mm long silver rod coated with palladium on which ^{125}I is adsorbed and encapsulated in a hollow cylindrical 0.05 mm thick titanium tube. The external dimensions of the seed are 0.8 mm diameter and 4.75 mm length. Sharma et al.^{[10]} calculated TG43 parameters in a 30 cm diameter spherical water phantom for this source using MCNP Version 3.1^{[21]} MC code. The authors have calculated radial dose functions up to a distance of 5 cm and anisotropy function at r = 1, 2, 3, and 5 cm for polar angles from 0° to 90° at 10° interval. In another study, Sahoo et al.^{[22]} reported dose rate constant and radial dose functions (up to a distance of 10 cm) for this source using DORZnrc usercode^{[23]} of EGSnrc code system.^{[5]}
In the present study, TG43 dosimetric parameters such as airkerma strength, S_{k}, dose rate constant, Λ, radial dose function, g(r) and anisotropy function, F(r,θ) and twodimensional (2D) absorbed dose rate data (alongaway table) are calculated for ^{192}Ir HDR VariSource VS2000,^{[8]} ^{169}Yb HDR 4140^{[9]}, and ^{125}I LDR OcuProsta^{[10]} brachytherapy sources using the new usercode egs_brachy^{[3],[4]} of the EGSnrc code system.^{[5]} Statdose^{[24]} and 3ddose_tools^{[25]} usercodes of EGSnrc code system are used for analyzing the dose distributions obtained from the egs_brachy MC code. The TG43 parameters calculated using egs_brachy are compared with the published data.^{[10],[14],[22]} For OcuProsta source, F(r,θ) are calculated for additional radial distances r = 0.25, 0.5, 7.5, and 10 cm for polar angles 0°–90° at an interval of 2°–5°. 2Ddose rate data (alongaway table) is also calculated in this study which is not available for this source.
Materials and Methods   
Egs_brachy Monte Carlo code
MCbased EGSnrc code system^{[5]} consists of several usercodes^{[23]} dedicated to address specific applications. egs_brachy^{[3],[4]} is a fast and versatile new usercode of EGSnrc code system designed especially for brachytherapy applications. egs_brachy is a modern EGSnrc application which employs C++ class library (egs++)^{[26]} for modeling geometries and particle sources.
Brachytherapy sources
Brachytherapy sources investigated in this study were HDR ^{192}Ir VariSource (Model VS2000),^{[8]} ^{169}Yb HDR (Model 4140)^{[9]} and ^{125}I LDR (Model OcuProsta).^{[10]} The geometry, dimensions, and material details of the above sources were taken from the published studies.^{[8],[9],[10]} The photon energy spectra of ^{192}Ir and ^{169}Yb needed for the MC calculations were taken from literature.^{[9],[27]} For ^{125}I source, the photon spectrum was taken from AAPM TG43U1.^{[2]}
Monte Carlo calculations
In the MC calculations of absorbed dose to water, the brachytherapy source was positioned at the center of the water phantom of mass density 0.998 g/cm^{3}. For ^{192}Ir VS2000^{[8]} and ^{169}Yb 4140^{[9]} sources, a cylindrical water phantom of dimensions 80 cm diameter and 80 cm height was simulated which was consistent with the recommendation of AAPM and ESTRO Report for photonemitting brachytherapy sources with an average energy higher than 50 keV.^{[28],[29]} For OcuProsta^{[10]} source, a cylindrical water phantom of dimensions 30 cm diameter and 30 cm height was considered which was consistent with the AAPM TG43U1 recommendations.^{[2]} The geometric center of the active part of the source was taken as the origin. The water phantom was divided into a number of cylindrical voxels with different sizes. For highdose gradients regions, small voxel sizes were adapted. Absorbed dose was scored in voxels of dimensions 0.1 mm × 0.1 mm for distance r ≤ 1 cm, 0.5 mm × 0.5 mm voxels for 1<r ≤5 cm, 1 mm × 1 mm voxels for 5<r ≤10 cm, and 2 mm × 2 mm voxels for 10< r ≤ 20 cm. For S_{k} calculations, the source was immersed at the center of a 50 cm diameter vacuum sphere. Airkerma per history was calculated in a voxel of dimension 0.1 cm × 0.1 cm × 0.05 cm filled with air (40% humidity, as recommended by TG43U1^{[2]}) located at a distance of 10 cm from the transverse axis of the source.
The PEGS4 dataset needed for MC calculations is based on the XCOM^{[30]} compilations. For the investigated sources, charged particle equilibrium was assumed and collisionkerma was considered as absorbed dose since the range of secondary electrons is short.^{[4]} The photon fluence spectrum scored using track length estimator was converted to collisionkerma to water by using the mass energyabsorption coefficients of water. Up to 8 × 10^{9} photon histories were simulated. Uncertainties were calculated with the default historybyhistory method used in EGSnrc code system.^{[31]} As per the recommendations of AAPM TG268,^{[32]} [Table 1] summarizes the parameters used in the MC calculations.  Table 1: Summary of parameters used for Monte Carlo calculations as per the recommendations of American Association of Physicists in Medicine task group286
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Results and Discussion   
Airkerma strength, S_{k}
The MCcalculated airkerma per history obtained at 10 cm was corrected to give the airkerma per history at a point of 1 m. The values of S_{k} calculated for ^{192}Ir VS2000, ^{169}Yb 4140, and ^{125}I OcuProsta sources are 1.202 ± 0.0014 × 10^{–13}, 2.184 ± 0.0019 × 10^{–14}, and 4.138 ± 0.0017 × 10^{–14} Gy cm^{2}/history, respectively.
Dose rate constant, Λ
The dose rate constant (Λ) was calculated by dividing the absorbed dose to water per history at reference position (1 cm, 90°) in the water phantom to the S_{k} per history. The values of Λ for ^{192}Ir VS2000, ^{169}Yb 4140 and ^{125}I OcuProsta sources are 1.099 ± 0.003, 1.186 ± 0.003, and 0.962 ± 0.003 cGyh^{1}U^{1}, respectively. The egs_barchycalculated values of dose rate constants are in excellent agreement with the published values^{[8],[9],[10],[14],[22]} within statistical uncertainties for all investigated sources.
Radial dose function, g(r)
Radial dose function, g(r), calculated for ^{192}Ir VS2000 and ^{169}Yb 4140 sources for distances r = 0.25–20 cm were presented in [Table 2] along with the corresponding published values.^{[14]} For ^{125}I OcuProsta source, g(r) values are calculated up to a distance of 10 cm and are presented in [Figure 1] along with the corresponding published values.^{[10],[22]} g(r) values for ^{192}Ir VS2000 source were found to be in good agreement with the published values^{[14]} with a maximum deviation of about 1.2% at a distance r = 18 cm. However, significant differences in g(r) values were observed beyond r = 8 cm which increases gradually with r when compared with the g(r) values (a maximum difference of about 28% at r = 18 cm) calculated by Angelopoulos et al.^{[8]} This is due to the fact that Angelopoulos et al.^{[8]} considered spherical water phantom of 40 cm diameter in their study whereas in the present study a cylindrical phantom of 80 cm diameter ×80 cm height is considered. The phantom dimensions significantly affect g(r) values only near the phantom boundaries. This effect is due to the reduction of scatter contribution to overall dose at the edges of the phantom.  Table 2: Radial dose function, g (r), of ^{192}Ir VS2000 and ^{169}Yb 4140 highdoserate brachytherapy sources
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 Figure 1: Radial dose function, g(r), of ^{125}I LDR OcuProsta brachytherapy source for radial distances 0.25–10 cm. The calculated data are based on a cylindrical liquid water phantom of dimensions 30 cm diameter ×30 cm height
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For ^{169}Yb 4140 and ^{125}I OccuProsta sources, excellent agreement is found between the g(r) values calculated using egs_brachy and the published values.^{[14],[22]} A maximum deviations of about 1.8% at a distance r = 20 cm and 0.68% at a distance r = 10 cm are observed for ^{169}Yb 4140 and ^{125}I OcuProsta sources, respectively.
Anisotropy function, F(r,θ)
For ^{192}Ir VS2000, ^{169}Yb 4140 sources, F(r,θ) were calculated at radii of 0.25, 0.5, 0.75, 1, 2, 3, 4, 5, 7.5, 10, 12.5, and 15 cm for polar angles from 0° to 180° with varying intervals. For ^{125}I OcuProsta source, F(r,θ) were calculated for polar angles 0° to 90° at radii of 0.25, 0.5, 0.75, 1, 2, 3, 4, 5, 7.5, and 10 cm because of the symmetry of the source. [Table 3], [Table 4], [Table 5] present the F(r,θ) values for ^{192}Ir VS2000, ^{169}Yb 4140, and ^{125}I OcuProsta sources, respectively. For ^{192}Ir VS2000 and ^{169}Yb 4140 sources, values of F(r,θ) are in good agreement with the published values^{[14]}  Table 3: Anisotropy function, F(r,θ), of ^{192}Ir VS2000 highdoserate brachytherapy source
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 Table 4: Anisotropy function, F(r,θ), of ^{169}Yb 4140 highdose rate brachytherapy source
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 Table 5: Anisotropy function, F(r,θ), of ^{125}I OcuProsta lowdose rate brachytherapy source
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For ^{125}I OcuProsta source, the values of F(r,θ) are in good agreement with the published values.^{[10]} [Figure 2]a and [Figure 2]b presents the values of F(r,θ) at different polar angles along with the corresponding published values^{[10]} at radial distances r = 1 and 5 cm, respectively.  Figure 2: (a) Anisotropy function, F(r,θ), of ^{125}I LDR OcuProsta brachytherapy source at a radial distance of 1 cm. The calculated data are based on a cylindrical liquid water phantom of dimensions 30 cm diameter × 30 cm height. (b) Anisotropy Function, F(r,θ), of ^{125}I LDR OcuProsta brachytherapy source at a radial distance of 5 cm. The calculated data are based on a cylindrical liquid water phantom of dimensions 30 cm diameter × 30 cm height
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Along and away twodimensional dose rate distribution
For ^{125}I OcuProsta source absorbed dose per unit airkerma strength was calculated up to a distance of 10 cm and presented in [Table 6]. The 2Ddose rate values for ^{192}Ir VS 2000 and ^{169}Yb 4140 sources agree well with the published data^{[14]} within 2%. It may be noted that, for ^{125}I OcuProsta source, 2D alongaway table is not available for comparison.  Table 6: Dose rate (2D along away) data per unit airkerma strength (cGyh^{−1} U^{−1}) for ^{125}I OcuProsta lowdoserate brachytherapy source
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Uncertainties
The uncertainties associated with the estimated quantities are only statistical. It does not include Type B uncertainty related to crosssection, source geometry, source material, and size of the voxel. However, to minimize the uncertainty that may arise due to dimensions of voxel, distancespecific voxel dimensions were chosen as recommended by Taylor et al.^{[15]} In this study, 1 σ statistical uncertainties on the calculated dosimetry values are <1% at distances r < 10 cm, <2% at distances r = 10–15 cm, and <3% at distances r = 10–20 cm.
Conclusion   
In this study, TG43 dosimetric parameters were calculated for ^{192}Ir VS2000, ^{169}Yb 4140, and ^{125}I Ocuprosta brachytherapy sources using the new egs_brachy usercode of the EGSnrc code system. The calculated dosimetric parameters are in good agreement with the published data. The present study validates the new usercode egs_brachy with the published dose distributions. This study thus demonstrates the ability of egs_brachy MC code to handle the transport of photons and electrons accurately at brachytherapy photon energies such as ^{192}Ir, ^{169}Yb, and ^{125}I. The study also demonstrates the capability of the egs_brachy to model the complex geometry of sources accurately. For example, the simulation of VS2000 which consists of two cylindrical sources having spherical caps at both ends, which is not possible using usercode such as DOSRZnrc due to the limitations associated with it.
Ethical approval
This article does not contain any studies with human participants or animals performed.
Informed consent
Informed consent was obtained from all individual participants included in this study.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]
