Monte Carlo study of TG-43 dosimetry parameters of GammaMed Plus high dose rate Ir brachytherapy source using TOPAS.

Purpose: To develop a simulation model for GammaMed Plus high dose rate Ir brachytherapy source in TOPAS Monte Carlo software and validate it by calculating the TG-43 dosimetry parameters and comparing them with published data.

Methods: We built a model for GammaMed Plus high dose rate brachytherapy source in TOPAS. The TG-43 dosimetry parameters including air-kerma strength S , dose-rate constant Λ, radial dose function g (r), and 2D anisotropy function F(r,θ) were calculated using Monte Carlo simulation with Geant4 physics models and NNDC Ir spectrum. Calculations using an old Ir spectrum were also carried out to evaluate the impact of incident spectrum and cross sections. The results were compared with published data.

Results: For calculations using the NNDC spectrum, the air-kerma strength per unit source activity S /A and Λ were 1.0139 × 10 U/Bq and 1.1101 cGy.h .U , which were 3.56% higher and 0.62% lower than the reference values, respectively. The g (r) agreed with reference values within 1% for radial distances from 2 mm to 20 cm. For radial distances of 1, 3, 5, and 10 cm, the agreements between F(r,θ) from this work and the reference data were within 1.5% for 15° < θ < 165°, and within 4% for all θ values. The discrepancies were attributed to the updated source spectrum and cross sections. They caused deviations of the S /A of 2.90% and 0.64%, respectively. As for g (r), they caused average deviations of -0.22% and 0.48%, respectively. Their impact on F(r,θ) was not quantified for the relatively high statistical uncertainties, but basically they did not result in significant discrepancies.

Conclusion: A model for GammaMed Plus high dose rate Ir brachytherapy source was developed in TOPAS and validated following TG-43 protocols, which can be used for future studies. The impact of updated incident spectrum and cross sections on the dosimetry parameters was quantified.