Dosimetric characterization of a new design (103)palladium brachytherapy source

Low-energy gamma emitting isotopes encapsulated for permanent implant are i n routine use in the treatment of prostate cancer. New source designs requi re full dosimetric analysis and calibration standardization before responsi ble clinical application. The results of one such experimental measurement and analysis are here reported for a new design of (103)palladium source, m odel MED3633 (North American Scientific, Inc.). By AAMP Task Group #43 reco mmendations, the reference material for brachytherapy dosimetry is liquid w ater. The dose measurements were made using standard methods employing ther moluminescent dosimeters in a water equivalent plastic phantom. Precision m achined bores in the phantom located dosimeters and source(s) in a reproduc ible fixed geometry providing for transverse-axis and angular dose profiles over a range of distances from 0.17 to 7 cm. The data were analyzed in ter ms of parameters recommended by AAPM TG43. The dose-rate constant, Lambda, was evaluated with reference to a (60)cobalt standard, accounting for respo nse variation with isotope energy spectrum. The radial dose function, g(r), the anisotropy function, F(r,theta), the anisotropy factor, phi(an)(r), an d the point-source approximation anisotropy constant, phi(an) were derived from one- and two-dimensional dose distribution data measured in the phanto m, accounting for finite dosimeter Volume and with attention to interchip e ffects. The results are compared to TG43 and other existing data for Pd-103 sources. The new source is comparable to the model 200 Pd-103 source desig n, demonstrating equivalent radial dose function, g(r). The dose surroundin g a MED3633 source may be slightly more isotropic than for the model 200 so urce. The air-kerma strength of the MED3633 source used in this study was p rovided by the manufacturer and is traceable to the NIST 1999 standard. The evaluated dose-rate constant, Lambda: with NIST traceable strength calibra tion is lower than that of the model 200 source, with that manufacturer's s trength calibration. (C) 1999 American Association of Physicists in Medicin e. [S0094-2405(99)00911-6].