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

Accurate dose calculation is essential in brachytherapy and frequently relies on Monte Carlo (MC) simulations for the determination of TG-43 dosimetric parameters. This study presents a comprehensive intercomparison of three general-purpose MC codes, MCNP6.2, TOPAS 3.9, and PHITS 3.341, for the evaluation of the geometry function, radial dose function, and anisotropy function of the microSelectron-HDR v2¹⁹²Ir source and the Theragenics-AgX100¹²⁵I seed. The simulations employed the complete photon emission spectra recommended by ICRP 107 and the datasets were rigorously benchmarked against established consensus literature, and a fifth-order polynomial fitting was applied to the radial dose function data. The results demonstrated excellent overall agreement among the three MC codes and the reference datasets. The polynomial fitting accurately reproduced the radial dose distributions, with deviations remaining predominantly below the recommended 2 % limit within clinically relevant radial intervals. Nevertheless, larger discrepancies were observed for the low-energy¹²⁵I source at extended distances, emphasizing the greater sensitivity of low-energy photon transport and attenuation effects compared with the more stable behavior observed for the¹⁹²Ir source. Overall, this study validates the accuracy and reliability of the investigated MC codes for high-precision brachytherapy dosimetry, establishing a robust computational baseline for future applications in patient-specific treatment planning and radiosensitization studies.