

ORIGINAL ARTICLE 



Year : 2012  Volume
: 37
 Issue : 4  Page : 219225 

Dosimetric comparison between the microSelectron HDR ^{192} Ir v2 source and the BEBIG ^{60} Co source for HDR brachytherapy using the EGSnrc Monte Carlo transport code
M Anwarul Islam^{1}, MM Akramuzzaman^{1}, GA Zakaria^{2}
^{1} Department of Physics, Jahangirnagar University, Savar, Dhaka, Bangladesh ^{2} Department of Medical Physics, Gummersbach Hospital, Academic Teaching Hospital of the University of Cologne, Germany
Date of Submission  18Jun2012 
Date of Decision  14Aug2012 
Date of Acceptance  08Oct2012 
Date of Web Publication  20Nov2012 
Correspondence Address: M Anwarul Islam Medical Physicist, Oncology and Radiotherapy Centre, SQUARE Hospitals Ltd, 18/F, West Panthapath, Dhaka  1205 Bangladesh
Source of Support: A part of this work has been done in Heidelberg under the collaboration program between Heidelberg University, Germany and Gono University, Bangladesh with the financial support of the German Academic Exchange Service (DAAD), Conflict of Interest: None  Check 
DOI: 10.4103/09716203.103608
Abstract   
Manufacturing of miniaturized high activity ^{192} Ir sources have been made a market preference in modern brachytherapy. The smaller dimensions of the sources are flexible for smaller diameter of the applicators and it is also suitable for interstitial implants. Presently, miniaturized ^{60} Co HDR sources have been made available with identical dimensions to those of ^{192} Ir sources. ^{60} Co sources have an advantage of longer half life while comparing with ^{192} Ir source. High dose rate brachytherapy sources with longer half life are logically pragmatic solution for developing country in economic point of view. This study is aimed to compare the TG43U1 dosimetric parameters for new BEBIG ^{60} Co HDR and new microSelectron ^{192} Ir HDR sources. Dosimetric parameters are calculated using EGSnrcbased Monte Carlo simulation code accordance with the AAPM TG43 formalism for microSlectron HDR ^{192} Ir v2 and new BEBIG ^{60} Co HDR sources. Airkerma strength per unit source activity, calculated in dry air are 9.698×10 ^{8} ± 0.55% U Bq ^{1} and 3.039×10 ^{7} ± 0.41% U Bq ^{1} for the above mentioned two sources, respectively. The calculated dose rate constants per unit airkerma strength in water medium are 1.116±0.12% cGy h ^{1} U ^{1} and 1.097±0.12% cGy h ^{1} U ^{1} , respectively, for the two sources. The values of radial dose function for distances up to 1 cm and more than 22 cm for BEBIG ^{60} Co HDR source are higher than that of other source. The anisotropic values are sharply increased to the longitudinal sides of the BEBIG ^{60} Co source and the rise is comparatively sharper than that of the other source. Tissue dependence of the absorbed dose has been investigated with vacuum phantom for breast, compact bone, blood, lung, thyroid, soft tissue, testis, and muscle. No significant variation is noted at 5 cm of radial distance in this regard while comparing the two sources except for lung tissues. The true dose rates are calculated with considering photon as well as electron transport using appropriate cutoff energy. No significant advantages or disadvantages are found in dosimetric aspect comparing with two sources.
Keywords: BEBIG Co60 HDR source, comparison of Ir192 and Co60, EGSnrc, HDR brachytherapy, microslectron Ir192, Monte Carlo code
How to cite this article: Islam M A, Akramuzzaman M M, Zakaria G A. Dosimetric comparison between the microSelectron HDR ^{192} Ir v2 source and the BEBIG ^{60} Co source for HDR brachytherapy using the EGSnrc Monte Carlo transport code. J Med Phys 2012;37:21925 
How to cite this URL: Islam M A, Akramuzzaman M M, Zakaria G A. Dosimetric comparison between the microSelectron HDR ^{192} Ir v2 source and the BEBIG ^{60} Co source for HDR brachytherapy using the EGSnrc Monte Carlo transport code. J Med Phys [serial online] 2012 [cited 2021 Apr 11];37:21925. Available from: https://www.jmp.org.in/text.asp?2012/37/4/219/103608 
Introduction   
This study comprises EGSnrc ^{[1]} Monte Carlo based dosimetry in accordance with AAPM TG43U1 ^{[2]} protocol for new BEBIG ^{60} Co HDR source model Co0. A86 (Eckert and Ziegler BEBIG GmbH, Germany) and microSelectron HDR ^{192} Ir source model 105.002 "v2" (Nucletron, Netherlands). The aim of this study was to compare the various dosimetry parameters as well as TG 43U1 ^{[2]} parameters for two sources. Meanwhile, some authors have published the relevant dosimetry data with different methodology for both sources. Richter et al. ^{[3]} have reported a comparison of ^{60} Co and ^{192} Ir sources using EGSRay ^{[4]} Monte Carlo based calculations and only photon emission has been considered for the simulations. Recently Selvam et al. ^{[5]} have published EGSnrc ^{[1]} Monte Carlo based dosimetry data except anisotropy function for BEBIG ^{60} Co HDR source. Moreover, Ballester et al. ^{[6]} and Granero et al. ^{[7]} have reported GEANT4 based Monte Carlo dosimetry data in accordance with TG43 ^{[2]} formalism for the same source. On the other hand, Daskalov et al. ^{[8]} have published dosimetry data for microSelectron HDR ^{192} Ir sources using MCPT Monte Carlo transport code. Collision kerma is used to approximate the absorbed dose calculation. Furthermore, Borg et al. ^{[9]} and Taylor et al. ^{[10],[11]} have reported TG43 dosimetry parameters for same source using BrachyDose ^{[12]} EGSnrc based Monte Carlo transport code.
In this work, the EGSnrc ^{[1]} Monte Carlo code system is used to calculate the TG43 ^{[2]} parameters with similar methodology for two sources. The true dose rates are calculated with considering photon as well as electron transport. The calculated radial dose function and anisotropy function values for both sources are compared. The calculated airkerma strength and dose rate constant are compared with other published data for the sources. The absorbed dose is calculated in different tissues related to the same dose in water for both sources and the results are compared to estimate the relative heterogeneity effect.
Materials and Methods   
Monte Carlo models of microSelectronHDR ^{192} Ir v2 source and BEBIG ^{60} Co HDR source
The microSelectron HDR ^{192} Ir source consists of pure iridium metal having density 22.39 g cm ^{3} . A source cylinder having diameter 0.065 cm and length 0.36 cm contains the radioactive ^{192} Ir material distributed uniformly inside it. The source core is encapsulated with an AISI 304 steel capsule which has outer diameter 0.09 cm, inner diameter 0.065 cm, and length 0.45 cm and is connected to a 0.2 cm long steel cable having diameter of 0.07 cm. The thickness of the capsule on the opposite end of the steel cable is 0.02 cm and side of the iridium core toward the cable is 0.07 cm. ^{[9]} [Figure 1]a shows the geometry of the real microSelectronHDR ^{192} Ir v2 source whose model used in the Monte Carlo calculations is shown in [Figure 1]b. The cylindrical geometric model is an approximation of the real geometry of the sources.  Figure 1: (a) Schematic diagram of the microSelectronHDR Ir192 v2 source and (b) the model diagram of the source used in Monte Carlo calculations. Dimensions shown are in centimetres and not to be scale
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The BEBIG ^{60} Co HDR source consists of pure cobalt metal (density of 8.9 g cm ^{3} ) and is kept inside the source cylinder having diameter 0.05 cm and length 0.35 cm. Radioactive ^{60} Co material is uniformly distributed inside it. As of the previous case the source core is encapsulated, but now with an AISI 316L steel capsule (outer diameter = 0.1 cm, inner diameter = 0.07 cm). The capsule is 0.5 cm long and connected to a 0.2 cm long steel cable. The capsule thickness is 0.075 cm on the longitudinal sides of the ^{60} Co source core and the thickness of axial side is 0.015 cm. There is an air gap of 0.01cm around the axial side of the active source core. ^{[13]} [Figure 2](a) shows the geometry of the real BEBIG ^{60} Co HDR source and [Figure 2](b) shows the model of it used in the Monte Carlo simulations.  Figure 2: (a) Schematic diagram of the new BEBIG Co60 HDR source and (b) the model diagram of the source used in Monte Carlo calculations. Dimensions shown are in centimetres and not to be scale
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Medium
For the calculation of airkerma strengths the sources are positioned at the center of a cylindrical container having 200 cm diameter and in dry air of mass density 0.001205 g cm ^{3} (pressure = 1 atm. and temperature = 20°C). Following the recommendation of TG43 ^{2} for absorbed dose calculations, the density of water was taken as 0.998 g cm ^{3} at 22°C. Several tissues are used as media for absorbed dose calculations. The ICRU density correction files ^{[14]} are used to make the pegs4 input files. Some human tissue equivalent materials are shown in [Table 1] are simulated to investigate the absorbed dose difference for BEBIG ^{60} Co and microSelectron HDR v2 sources.  Table 1: Human equivalent tissue materials which are used in Monte Carlo simulation for the calculation of tissue dependent absorbed dose for the sources
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Monte Carlo simulations
The EGSnrc code system ^{[1]} used in the present work is widely accepted Monte Carlo simulation code for coupled electronphoton transport. Its current energy range of applicability is considered to be 1 keV to 10 GeV. It is an extended and improved version of the EGS4 package ^{[1]} originally developed at Stanford Linear Accelerator Center (SLAC). ^{[15]} In particular, it incorporates significant improvements in the implementation of the condensed history technique for the simulation of charged particle transport and better low energy cross sections. ^{[14]} In this study, the latest version of the code is used for calculations of photon fluence and absorbed dose to water.
The halflife for ^{192} Ir is 73.825 days and as an average onedecay will result in the emission of 1 electron and 2.363 photons. ^{[9]} The decay of ^{192} Ir radionuclide occurs through 4.7 % electron capture and 95.3 % β^{} transitions followed by γ transitions and k and Lshells Xrays. ^{[9]} " ^{192} Ir_ bare_1993" spectrum and " ^{192} Ir_beta" spectrum are used as input photon spectrum and as beta spectrum respectively presented by Duchemin et al. ^{[16]} for all Monte Carlo calculations of microSelectron_HDR ^{192} Ir source. The halflife of ^{60} Co radionuclide is 5.27 years and onedecay will result of emission of two photon spectrums with energy of 1.17 MeV and 1.33 MeV. ^{[13]} The "bareco60" spectra file is used for all the subsequent Monte Carlo calculations of BEBIG ^{60} Co HDR source. The number of history used for all calculations is 10 ^{9.} The bound Compton scattering, PE angular sampling, Raleigh scattering and atomic relaxations are considered as transport parameters. Variance reduction techniques are avoided in the calculations.
Calculation of airkerma strength
Airkerma strength, S _{k} is a measure of brachytherapy source strength which is specified in terms of airkerma rate, at a point along the transverse axis of the source in free space. ^{[2]} It is defined as the product of airkerma rate, at a calibration distance, d, in free space, measured along the transverse bisector of the source and the square of the distance, d: ^{[9]}
The total airkerma is related to the photon fluence by
where, φ(E) [MeV ^{1} cm ^{2} ] is the photon fluence per unit energy at energy E [MeV] and is the mass energyabsorption coefficient at the same energy E. The factor 1.602 × 10 ^{10} is required to convert K _{air} from MeV g ^{1} into Gy.
The total airkerma is calculated from the equation below ^{[9]} :
Here, E _{i} is the midpoint of each energy bin and ∆E is the bin size. The airkerma rate K_{air} in [Gy/s] of source activity A in [Bq] and number of photon per decay, N _{p} is determined from:
The airkerma strength per unit source activity is then calculated using equation (1) and dividing it by the activity A:
Here, the unit of S _{k} is denoted by the symbol U, and U = 1 μGy m ^{2} h ^{1 =} 1 cGy cm ^{2} h ^{1} .
The airkerma per initial particle is calculated at a distance 100 cm from the center of the source as per AAPM TG43 recommendation inside dry air medium. ^{[2]} The mass energyabsorption coefficient, for dry air are taken from the latest NIST compilation. ^{[17]} The usercode FLURZnrc ^{[1]} is used to calculate the differential fluence spectrum, φ′(E) in the calculation grid per initial photon in the simulation. The photon fluence spectrum in 5 keV ^{[2]} intervals was scored along the transverse axis for the point of 100 cm distance. The cutoff energy for photon and electron transport are 0.001 MeV and 2 MeV. In the present work, bare iridium source data (Ir192_bare_1993. spectrum) is used as spectra file for simulation to calculate the differential fluence spectrum for microSelectron HDR v2 source. The beta spectrum is not considered in the calculation of airkerma strength in accordance with the TG43 protocol. ^{[2]} But both the spectra are used in the dose rate calculations for the source. Similarly, bare cobalt60 spectrum (bareco60. spectrum) is used to simulate for the BEBIG ^{60} Co source in the calculation of airkerma strength and also dose rate.
To estimate the airkerma strength, the source is kept in a 2 × 2 × 2 m ^{3} air cylinder and kerma is scored for a 0.2 cm thick and 0.1 cm high cylindrical ring cell, located along the transverse source axis. The number of 10 ^{[9]} histories is simulated to obtain airkerma strength for both sources. Whereas Borg et al. ^{[9]} calculated airkerma strength as same methodology for 50 cm distance for microSelectron HDR ^{192} Ir v2 source. Selvam et al. ^{[5]} have calculated using same user code with photon fluence spectrum in 10 keV interval at 100 cm distance in a 1 cm thick and 0.5 cm height of voxel size for new BEBIG ^{60} Co HDR source. Moreover, Sahoo et al. ^{[13]} calculated the airkerma strength at 100 cm distance using MCNP code for new BEBIG ^{60} Co HDR source.
Calculations of absorbed dose rate
For the absorbed dose rate calculation the source is positioned at the centre of a cylindrical water phantom of volume π × 1 ^{2} × 2 m ^{3} . In order to provide adequate spatial resolution, the cells are 0.01 cm thickness for <2 cm, 0.05 cm for 2 cm < r < 5 cm, 0.1 cm for 5 cm < r < 10 cm and 0.2 cm for r > 10 cm from the source. ^{[11]} The dose rate values are calculated in different positions of the water phantom with polar coordinate for different position of the water phantom. The usercode DOSRZnrc ^{[1]} is used to calculate D _{photon} and D _{electron} for subsequent calculation of the true dose is using the equation (6): ^{[18]}
where D _{photon} is the total dose produced by photons, D _{electron} is that due to electrons, S k/A is airkerma strength per unit source activity in [U Bq ^{1} ]. The factor 3.6 × 10 ^{5} is required to convert the dose rate per unit airkerma strength per unit source activity in [cGy U ^{1} ] to the dose rate per unit airkerma strength. The values of N _{p} for ^{192} Ir and ^{60} Co are 2.363 ± 0.3% ^{9} and 2, respectively. ^{13} The true dose rate is calculated for all points of interest and these values are used to calculate TG43 parameters ^{[2]} e.g., dose rate constant, radial dose function and anisotropy function. The cutoff energy for photon and electron transport are 0.001 MeV and 0.521 MeV, respectively, as maintained in the dose rate calculations for all radial distances. Daskalov et al. ^{[8]} simulated for new microSelectron HDR ^{192} Ir source using MCPT based Monte Carlo transport code and collision kerma is used to approximate the absorbed dose, whereas secondary electron transport was not considered for calculation. Taylor et al. ^{[10],[11]} have reported TG43 parameter's value for same source using EGSnrc based BrachyDose ^{[12]} Monte Carlo transport code without considering electron transport. Granero et al. ^{[7]} and Richter et al. ^{[3]} calculated the dosimetry parameters using Monte Carlo based transport code GEANT4 and EGSRay, ^{[4]} respectively, for new BEBIG ^{60} Co HDR source and the authors used 10 keV cutoff energy in the simulation for photon and electron. Selvam et al. ^{[5]} calculated absorbed dose to water using DOSRZnrc user code ^{[1]} of EGSnrc for the same source with cutoff energy 0.001 MeV and 0.521 MeV for photon and electron transport, respectively. The authors also calculated collision kerma to approximate the absorbed dose close to the source (up to 1 cm) and electron rage rejection technique (ESAVE =2 MeV) is used for rest of the calculations.
Results and Discussion   
Airkerma strength, S _{k}
The calculated value of airkerma strength per unit source activity is found to be 9.698 × 10 ^{8} ± 0.55% U Bq ^{1} for new microSlectron HDR ^{192} Ir v2 source. The source is also simulated at the point of 100 cm distance from the source centre of a cylindrical air phantom. This value is in good agreement with results (=9.737 ± 0.009 × 10 ^{8} U Bq ^{1} ) obtained by Hong et al. ^{[18]} at an average value from 2 to 50 cm distances with 0.2% contribution from bremsstrahlung. The extracted results at 50 cm distance without bremsstrahlung contribution was 9.694 ± 0.037 × 10 ^{8} U Bq ^{1} . The results also agree well with the observed results (9.71 × 10 ^{8} ± 0.4% U Bq ^{1} ) of Borg and Rogers ^{[9]} for airkerma strength per unit source activity for the microSlectron HDR ^{192} Ir v2 source at 50 cm distance from the source centre. It's worth mentioning that thier methodology has been adopted in the present cases also.
The value of airkerma strength per unit source activity for BEBIG ^{60} Co HDR source is found to be 3.039 × 10 ^{7} ± 0.41% U Bq ^{1} . All the parameters are set as like the previous case of calculation of the microSlectron HDR ^{192} Ir v2 source. Selvam et al. ^{[5]} obtained the value (=3.043 × 10 ^{7} U Bq ^{1} ) using same code with 10 keV interval of fluence spectrum. The value is about 0.13% higher than the value obtained from this study probably due to higher size of calculation grid. In this case also the results are again in good agreement with Monte Carlo based MCNP results of 3.04 × 10 ^{7} ± 0.05% U Bq ^{1} by Sahoo et al. ^{[13]}
Dose rate constant, Λ
Dose rate constants, Λ, are calculated by dividing the dose to water per unit source activity in a (0.1 mm) ^{3} voxel centred at the reference position, (1 cm, 90°), in the π × 100 ^{2} × 200 cm ^{3} cylindrical water phantom, by the airkerma strength per unit source activity. The contribution of primary electron is accounted for in all the calculations related to microSlectron HDR ^{192} Ir v2 source. The value of Λ for microSlectron HDR ^{192} Ir v2 source is found to be 1.116 ± 0.12% cGy h ^{1} U ^{1} . This result agrees well with the result of Taylor et al. ^{[10]} (=1.109 ± 0.18% cGy h ^{1} U ^{1} ) and Hong et al. ^{[18]} (=1.112 ± 0.51% cGy h ^{1} U ^{1} ).
The calculated value of Λ for BEBIG ^{60} Co HDR source is found to be 1.097 ± 0.12% cGy h ^{1} U ^{1} whereas Selvam et al. ^{[5]} have reported the dose rate constant value (=1.097 cGy h ^{1} U ^{1} ) using FLURZnrcbased ^{[1]} calculation of collision kerma to approximate the dose. This is also in good agreement with GEANT4 Monte Carlo based published results of 1.087 ± 0.011% cGy h ^{1} U ^{} 1 by Granero et al. ^{[7]}
Radial dose function, g _{L} (r) and anisotropy function,G _{L}(r, θ)
The radial dose function, g _{L} (r) accounts for dose fall off on the transverseplane due to photon scattering and attenuation in water medium. The function is also influenced by the geometry factor, G _{L} (r, θ) and the anisotropy factor, F(r, θ). The geometry factor depends on the physical parameters of the source, i.g., the length and the radius of the source. An identical construction of of the sources can ensure same geometry factors. The isodose curve is influenced by the anisotropy factor in clinical dose distribution. These two functions are essential for comparing different brachytherapy sources.
The [Figure 3] shows the comparison of radial dose functions from 0.06 cm to 100 cm radial distance and the [Figure 4] does the same for distance from 0.06 cm to 2 cm. The values of radial dose function for ^{60} Co source is about 2.4% lower than ^{192} Ir at 2 cm radial distance (Manchester Point A) and the values are also found to be lower 7.5%, 9.3% and 10.3% for the point of 5 cm, 7.5 cm and 10 cm respectively. The values of radial dose function of ^{60} Co source are also lower than that of the ^{192} Ir source in the range from 0.06 cm 0.17 cm. The values are, however, found higher in the range from 0.18 cm 1 cm and above 22 cm of radial distance. The higher values of radial dose function are found for ^{192} Ir source at the distance shorter than 0.17 cm may be the dose by electron contribution. The values are found to be less by 7% averagely using ECUT =2 MeV. ^{[19]} The radial dose function for ^{60} Co source is linearly falloff from 1 cm to 7.5 cm of radial distances.
The anisotropy factors of BEBIG ^{60} Co HDR source are comparatively higher at the longitudinal sides of the source with the values of microSelectron HDR ^{192} Ir source. It is also sharply increases with the radial distances to the longitudinal side of the BEBIG ^{60} Co source comparatively with the value of microSlectron ^{192} Ir v2 source. These factors for the cable connecting side of the BEBIG ^{60} Co source are quite low compared to that of the other side and the values show an increase with the radial distance. The [Figure 5],[Figure 6],[Figure 7] and [Figure 8] compare the anisotropy factors for 2 sources at radial distances 1 cm, 3 cm, 5 cm and 10 cm.  Figure 3: Comparison of radial dose function for microSelectron HDR ^{192}Ir and BEBIG ^{60}Co HDR sources from 0.06 cm to 100 cm of radial distances
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 Figure 4: Comparison of radial dose function for microSelectron HDR ^{192}Ir and BEBIG ^{60}Co HDR sources from 0.06 cm to 2 cm of radial distances
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 Figure 8: Comparison of anisotropy function for 10 cm of radial distance
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Absorbed dose difference
A cylindrical phantom was constructed with the respective tissue equivalent materials [Table 1] for simulation. The absorbed doses are calculated at different radial distances. The absorbed dose of water at discrete distances has been used as reference dose for the calculation of relative dose differences. The [Figure 9] shows the relative absorbed dose differences between ^{60} Co and ^{192} Ir sources. Significant dose differences are noted for compact bone tissue up to 8 cm and more than 10 cm radial distances. Maximal absorbed dose differences are, however, observed in lung tissue: The results being about 30.8% at 0.2 cm, and 12.45% at 1 cm. As is expected the difference decreases with increase in radial distances. Tissue dependence of absorbed dose has been calculated with vacuum phantom for water, breast, compact bone, blood, lung, thyroid, soft tissue, testis and muscle. No significant tissue dependence in absorbed dose is noted at 5 cm of radial distance by comparing BEBIG ^{60} Co HDR to microSlectron HDR ^{192} Ir v2 sources except for the lung tissues. [Figure 10] shows the differences comparing them with the absorbed dose of water.  Figure 9: Relative absorbed dose difference for compact bone and lung with water
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 Figure 10: Relative absorbed dose difference for deferent tissue materials at 5 cm radial distance
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The investigations of ^{60} Co and ^{192} Ir sources have shown approximately identical dose distribution. Negligible differences are noted in radial dose distribution as well as negligible absorb dose differences with various tissues. A significant difference appears in absorbed dose at the close surface of the iridium source (132 times higher than the dose of 1 cm of radial distance from the source centre) ^{[19]} probably due to the contribution of primary electron. These differences are minimal when the calculation is done with KERMA approximation. ^{[19]} In case of ^{60} Co source, the absorbed dose is about 74 times higher ^{[19]} than the dose of 1 cm of radial distance.
Conclusions   
The values of anisotropy function at the longitudinal sides of the BEBIG ^{60} Co HDR source are relatively higher than microSelectron HDR ^{192} Ir source and the isodose lines will be not exactly identical due to this effect for these region. Treatment planning system performs to adjust the isodose distribution using dwell time positioning technology in clinical relevant situation. The radial dose function characterize the dose falloff on the transverse plane of the source. The higher dose region for ^{192} Ir source and lower dose region for ^{60} Co source (<0.17 cm from the source centre) may be situated inside the applicators and rest of the region up to 1 cm, the radial dose falloff will be dependent on the nature of the applicator. The applicators commonly use for cervical site; these region again may be situated inside the applicators. The differences of the radial dose function <1 cm for 2 sources may be effectual for narrow catheter based intracavitary or interstitial brachytherapy. The small difference of radial dose function (within the therapeutic range of radial distance) has a possibility to make a negligible difference in isodose distribution in clinical applications.
Acknowledgements   
The authors are grateful to Prof. Dr. Günter H. Hartmann, Dr. Mario Parez, T. Palani Selvam and Liu Hong for their valuable technical support and encouragement.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10]
[Table 1]
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