Dosimetry of the I-Plant Model 3500 iodine-125 brachytherapy source.

125I brachytherapy sources have been widely used for interstitial implants for a number of years in several tumor sites, especially the prostate. The design of the new I-Plant Model 3500 iodine source is novel, yet its characteristics are similar to those of two existing designs, Model 6711 and the Symmetra. Dosimetry parameters (including dose rate constant, radial dose function, and anisotropy function, as defined by AAPM Task Group 43) were measured with LiF thermoluminescent dosimeters in water-equivalent plastic phantoms. The dose rate constant was found by direct comparison of calibrated I-Plant Model 3500 and Model 6711 seeds in a solid water phantom, to be 1.01 (cGy/h)/U. The radial dose function and anisotropy function are similar to those of the Model 6711 and Symmetra seeds.