Monte Carlo calculations of AAPM Task Group Report No. 43 dosimetry parameters for the MED3631-A/M125I source

Brachytherapy dosimetry parameters for MED3631-A/M 125I sources have been determined in accordance with the AAPM Task Group No. 43 (TG-43) dosimetry protocol. These data were calculated using the *F8 tally from the MCNP4B2 Monte Carlo radiation transport code with the DLC-189 cross-section libraries. Due to motion of the 125I resin beads and gold-copper markers within the capsule, parameters such as the geometry function, radial dose function, dose rate constant, and anisotropy function were examined with the beads and markers having either "realistic" or "ideal" positions; the realistic position was a weighted combination of "vertical" and "diagonal" capsule orientations. The dose rate constants for the realistic and ideal geometries, lambda99std(realistic) and lambda99std(ideal) were 1.066 and 1.067 cGy h(-1) U(-1), respectively, which were within uncertainties of measured values by Wallace and Fan [Med. Phys. 26, 1925-1931 (1999)] and Li et al. [Med. Phys. 27, 1275-1280 (2000)], 1.06 and 1.067 cGy h(-1) U(-1), respectively. The calculated reference dose rate at r0= 1 and theta0= 90 degrees for the realistic source geometry was 0.7% less than for the ideal source geometry. The anisotropy constants, phian(realistic), for the realistic and ideal geometries were 0.948 and 0.965, respectively. phian(realistic) matched that (0.941) measured by Wallace and Fan, and phian(ideal) was significantly different from that (0.948) calculated by Wierzbicki et al. [Med. Phys. 25, 2197-2199 (1998)] for an ideal MED3631-A/S 125I source.