Monte Carlo-aided dosimetry of the symmetra model I25,S06 I-125, interstitial brachytherapy seed

A dosimetric study of a new I-125 seed for permanent prostate implant, the Symmetra I-125 Seed model I25.S06, has been undertaken utilizing Monte Carl o photon transport calculations. All dosimetric quantities recommended by t he AAPM Task Group 43 (TG-43) report have been calculated. Quantities deter mined are dose rate constant, radial dose function, anisotropy function, an isotropy factor, and anisotropy constant. The recently (January 1999) revis ed NIST (National Institute of Standards and Technology) I-125 Standard for air kerma strength calibration was taken into account as well as updated i nteraction cross-section data. Calculations were done for the competing mod el 6702 source for the purpose of comparison. The calculated dose-rate cons tants for the two seeds are 1.010 and 1.016 cGyh(-1) U-1 for the Symmetra a nd model 6702 seeds, respectively. The latter value deviates from the value , 1.039 cGyh(-1) U-1, recommended in the TG-43 report. The calculated radia l dose function for the Symmetra new seed is more penetrating than that of the model 6711 seed (by 20% at 5 cm distance) but agrees closely (within st atistical errors) with that of the model 6702 seed up to distances of 10 cm . The anisotropy function for the seed is also close to that for the 6702 s eed with a tendency of somewhat more pronounced anisotropy (lower values at small angles from the longitudinal axis). Compared to the model 6711 seed, the Symmetra new seed is more isotropic. The anisotropy constants (the ani sotropy function averaged with respect to angle and distance) for the three seed models are within 2%. (C) 2000 American Association of Physicists in Medicine. [S0094-2405(00)01605-9].