Determination of American Association of Physicists in Medicine Task Group 43 dosimetry parameters for Best Industries model 81-02 gold-198 brachytherapy source and their relevance to patient dosimetry

The practice of introducing radioactive sources into or near tumors continues to be an important method of delivering radiation (Khan 1994). The American Association of Physicists in Medicine Task Group 43: Dosimetry of Interstitial Brachytherapy Sources (TG 43) (Nath 1995) recommends that specific parameters be determined for a brachytherapy source in order to provide an accurate, three-dimensional, description of the dose distribution. The dosimetric properties recommended by TG 43, the radial dose function the anisotropy function, and the dose rate constant were determined for the Best Medical Industries (Springfield, VA) model 81-02 Au-198 brachytherapy seed source. These parameters were determined by Monte Carlo simulation, verified by TLD measurement and other methods, and compared to previously published data. The MCNP4C code (Los Alamos National Laboratory) was used to simulate electron and photon transport from a cylindrical source modeled after the 81-02 seed. Source measurements were made with TLD rods in a Solid Water phantom. The radial dose functions and anisotropy functions determined by measurement and Monte Carlo simulation agreed with each other with an average deviation of less than 2%. The dose rate constant was determined using Monte Carlo simulation, TLD measurement, and two calculation methods (Chen Z, Nath R 2001). All methods, except the TLD measurement, agreed within 2% of the average value. This is most likely explained by the lack of a NIST traceable calibration standard for Au-198. The data from this study and previously published data were used for a planar implant to investigate the clinical impact of the new data. The resulting dose distributions showed a nominal 5% decrease in dose using the new TG-43 parameters.