Dosimetric characterization of an encapsulated interstitial brachytherapy source of 125I on a tungsten substrate

Purpose: Recently, a new design of an encapsulated 125I source using a tungsten substrate has been introduced by Best Medical International and named as Best Model 2301 source. In contrast to model 6711 source that uses silver as substrate, the model 2301 source does not yield fluorescent x rays (22.1 keV and 25.5 keV) in the energy range of dosimetric interest. This changes the dosimetric characteristics of the source and experimental determination of these characteristics is needed. Methods and Materials: In this work, the dosimetric characteristics of the tungstenbased 125I source were measured using LiF TLDs in a Solid Water™ phantom. The dose rate constant as well as the radial dose function and anisotropy function were measured. Results: The dose rate constant for the tungsten-based source was determined to be 1.02 ± 0.07 cGy h−1 U−1 in contrast to the previously reported value of 0.98 for the silver-based model 6711 source. The radial dose function for the tungsten-based model 2301 source decreases slightly less rapidly with distance than that for the silver-based model 6711 source. Considerable differences in the anisotropy functions between the two sources were observed. Conclusions: Dosimetric parameters of the Model 2301 source, based on AAPM TG-43 formalism, have been experimentally determined.