Monte Carlo calculations of AAPM Task Group Report No. 43 dosimetry parameters for the ¹²⁵I I-Seed AgX100 source model

Purpose: The purpose of this study was to determine the dosimetric parameters of the AgX100, a new ¹²⁵I brachytherapy seed model, using Monte Carlo (MC) simulations according to the protocol specified by the updated American Association of Physicists in Medicine Task Group No. 43 Report (TG-43U1) and compare these parameters with those of the established brachytherapy ¹²⁵I seed models 6711 and I25.S06. Methods and Materials: Independent verification of the new seed geometry was performed using high-resolution digital radiography and scanning electron microscopy. MCNPX v.2.5 MC simulations of the AgX100 seed were performed to derive its TG-43U1 parameters, the dose rate constant, the radial dose function, and the two- and one-dimensional anisotropy functions in liquid water. A dosimetric error propagation analysis was also performed to include uncertainty because of seed manufacturing tolerances and physics parameters. Results: The MC-calculated dose rate constant for the AgX100 seed was 0.943cGy·h ^-1·U ^-1±2.6% (k=1) based on the air kerma strength for a simulated point detector. Tabulated results of the radial dose function for line and point source approximations and the two-dimensional anisotropy function are also reported. Conclusions: The MC-predicted dose distribution of the AgX100 seed was found to be comparable with that of the model 6711 seed but much different from the dose distribution of the model I25.S06 seeds. However, at shallow distances, there were some dosimetric differences between the AgX100 and 6711 seed, which warrant separate TG-43U1 parameters for use in clinical treatment planning systems.