Monte Carlo Study on Dose Distributions Around Ir, Yb, and I Brachytherapy Sources Using EGSnrc-based egs_brachy User-code.

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 TG-43 dosimetric parameters calculated using the new MC user-code egs_brachy of EGSnrc code system for three different radionuclides Ir, Yb, and I which represent high-, intermediate-, and low-energy sources, respectively.

Materials And Methods: Brachytherapy sources investigated in this study are high-dose rate (HDR) Ir VariSource (Model VS2000), Yb HDR (Model 4140), and I -low-dose-rate (LDR) (Model OcuProsta). The TG-43 dosimetric parameters such as air-kerma strength, , dose rate constant, Λ, radial dose function, g(r) and anisotropy function, θ and two-dimensional (2D) absorbed dose rate data (along-away table) are calculated in a cylindrical water phantom of mass density 0.998 g/cm using the MC code egs_brachy. Dimensions of phantom considered for Ir VS2000 and Yb sources are 80 cm diameter ×80 cm height, whereas for 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 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 TG-43 dose parameters calculated using egs_brachy for Ir, Yb, and I brachytherapy sources with the values published in the literature.