Dosimetric characteristics of a new 125I brachytherapy source.

125I brachytherapy sources are being used for interstitial implants in tumor sites such as the prostate. Recently, a new 125I source has been introduced, which has a design different from that of other sources presently on the market. Dosimetric characteristics of this source, including dose rate constant, radial dose function, and anisotropy function, were determined experimentally following the AAPM Task Group 43 recommendations. The characteristics were related to the 1999 NIST calibration assigned to this source [SK,99std]. Measurements were performed in a solid water phantom using LiF thermoluminescent dosimeters. For these measurements, slabs of solid water phantom material were machined to accommodate the source and LiF TLD chips of dimensions (3.1 x 3.1 x 0.8 mm3) and (1.0 x 1.0 x 1.0 mm3). The TLD chips were surrounded by at least 10 cm of solid water phantom material to provide full scattering conditions. The results indicated a dose rate constant, lambda, of 0.88 +/- 0.07cGyh(-1)U(-1) for the new 1251 source as compared to 0.98 and 1.04 cGy h(-1)U(-1) for the Nycomed/Amersham model 6711 and 6702 seeds, respectively. Per TG-43, the values reported here represent the dose absorbed by water at 1 cm from the source in a water medium. The radial dose function, g(r), of the new 125I source was measured at distances ranging from 0.5 to 10 cm. The anisotropy function, F(r,theta), of the new 125I source was measured at distances of 2 and 5 cm from the source center. Calculations of anisotropy and radial dose function were also made using a Monte Carlo code. These calculations were made for both solid water and liquid water, the former to validate the Monte Carlo code and the latter to provide results in liquid water for clinical use. All data compared favorably with those from the Nycomed/Amersham models 6711 and 6702 sources.