Dosimetric characteristics of the Pharma Seed model BT-125-I source.

125I brachytherapy sources are being used with increasing frequency for interstitial implants in tumor sites, especially the prostate. Recently, a new 125I source design has become commercially available for clinical applications. Dosimetric characteristics (i.e., dose rate constant, radial dose function, and anisotropy function) of this source were experimentally and theoretically determined following the AAPM Task Group 43 (TG-43) recommendations and were related to the 1999 NIST calibration assigned to this source [S(k), 99std]. Measurements were performed in a Solid Water phantom using LiF thermoluminescent dosimeters. The measured data were used to validate the Monte Carlo simulations that were performed in Solid Water using the PTRAN code. The Monte Carlo calculations were then performed in liquid water to obtain the dosimetric information for clinical applications in accordance with TG-43 recommendations. The results indicated that the dose rate constant, lambda, of the Pharma Seed model BT-125-I 125I source was 0.90 +/- 0.06 cGy h(-1) U(-1) using thermoluminescent dosimeter (TLD) measurements and 0.92 +/- 0.03 cGy h(-1) U(-1) using Monte Carlo simulations in Solid Water. The calculated value in liquid water was found to be 0.95 +/- 0.03 cGy h(-1) U(-1). The radial dose function, g(r), of the new 125I source was measured at distances ranging from 0.5 to 10 cm using LiF TLD in Solid Water phantom material. The Monte Carlo simulations were performed for distances ranging from 0.1 to 10 cm from the source center in Solid Water and liquid water. The anisotropy function, F(r, theta), was measured at distances of 2, 5, and 7 cm from the source center and calculated at distances of 0.5, 1, 2, 3, 5, and 7 cm from the source center. The anisotropy constant, phi(an), of the Pharma Seed source in water was found to be 0.975. Complete dosimetric data are described in this manuscript. Per TG-43, the values reported in water should be used for clinical treatment planning systems.