Monte Carlo intercomparison of TG-43 dosimetric parameters for clinical192Ir and125I brachytherapy sources

Objective.This study presents a rigorous, simultaneous Monte Carlo (MC) intercomparison of three general-purpose computational codes, MCNP6.2, TOPAS 3.9, and PHITS 3.341, for the calculation and validation of TG-43 dosimetric parameters for the clinical microSelectron-HDR v2Ir and Theragenics-AgX100I brachytherapy sources under realistic, unsimplified photon emission spectra.Approach.Simulations were performed in a spherical liquid water phantom with a radius of 18.5 cm to ensure full-scatter conditions. Dose was scored in small, distance-dependent spherical water scoring cells with varying radii. Discrete, unsimplified photon emission spectra from ICRP Publication 107 were implemented in all three codes. To address gaps in the literature, a computationally efficient simplified geometry previously adopted for theIr source was evaluated, while theI source was modeled with high geometric fidelity. The radial dose and anisotropy functions were calculated, the radial dose data were fitted using fifth-degree polynomials, and the resulting datasets were benchmarked against established TG-43 consensus reference data.Main results.The three MC codes demonstrated good overall agreement. For theIr source, the simplified geometry reproduced the consensus reference data within thecriterion over the clinically relevant radial range, demonstrating its dosimetric suitability within the TG-43 framework. For the low-energyI source, larger deviations, reaching approximately, were observed at short radial distances (cm), consistent with the greater sensitivity of low-energy photon transport to attenuation and shielding effects associated with the source encapsulation and internal components. At distances beyond approximatelycm, the results showed substantially improved agreement, with deviations generally remaining within. Fifth-degree polynomial fitting accurately reproduced the radial dose function, with deviations predominantly remaining below the recommendedcriterion within the clinically relevant radial intervals.Significance.This work establishes a robust, cross-validated computational baseline for brachytherapy dosimetry using general-purpose MC codes under realistic photon emission spectra. The validation of the simplifiedIr geometry supports its use in computationally efficient simulations, while the characterization of the low-energyI source provides relevant information for high-precision dosimetry and future investigations of low-energy radiation interactions in radiosensitization studies.