Monte Carlo-aided dosimetry of the Source Tech Medical Model STM1251 I-125interstitial brachytherapy source

We have used Monte Carlo photon transport simulations to calculate the dosi metric parameters of a new I-125 seed, the Source TI-ch Medical Model STM12 51 source for interstitial brachytherapy. We followed the recommendations o f the AAPM Task Group 43 and determined the following parameters: dose-rate constant, radial dose function, anisotropy function, anisotropy factor, an d anisotropy constant. The recently (January 1999) revised National Institu te of Standards and Technology I-125 standard for air-kerma strength calibr ation was taken into account as well as updated interaction cross-section d ata. The calculated dose-rate constant, when normalized to the simulated wi de-angle, free-air chamber measurement of air-karma strength, is 0.980 cGy h-l U-l. The calculated radial dose function for the Model STM 1251 source is Inore penetrating than that of the model 6711 seed (by 18% at 5 cm dista nce), but agrees closely (within statistical errors) with that of the model 6702 seed up to distances of 10 cm. The STM 1251 source anisotropy functio ns indicate that its dose distribution is somewhat more anisotropic than th at of the model 6702 and 6711 seeds at 1 cm distance but is comparable at l arger distances. The Model STM1251 anisotropy constant is very similar to t hat of the model 6711, 6702, and MED3631 A/M seeds. (C) 2001 American Assoc iation of Physicists in Medicine.