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

We have used Monte Carlo photon transport simulations to calculate the dosimetric parameters of a new 125I seed, the Source Tech Medical Model STM125I source for interstitial brachytherapy. We followed the recommendations of the AAPM Task Group 43 and determined the following parameters: dose-rate constant, radial dose function, anisotropy function, anisotropy factor, and anisotropy constant. The recently (January 1999) revised National Institute of Standards and Technology I-125 standard for air-kerma strength calibration was taken into account as well as updated interaction cross-section data. The calculated dose-rate constant, when normalized to the simulated wide-angle, free-air chamber measurement of air-kerma strength, is 0.980 cGy h(-1) U(-1). The calculated radial dose function for the Model STM 1251 source is more penetrating than that of the model 6711 seed (by 18% at 5 cm distance), but agrees closely (within statistical errors) with that of the model 6702 seed up to distances of 10 cm. The STM125I source anisotropy functions indicate that its dose distribution is somewhat more anisotropic than that of the model 6702 and 6711 seeds at 1 cm distance but is comparable at larger distances. The Model STM125I anisotropy constant is very similar to that of the model 6711, 6702, and MED363I A/M seeds.