Dosimetric characterization of a new design 103 palladium brachytherapy source

Abstract

Low-energy gamma emitting isotopes encapsulated for permanent implant are in routine use in the treatment of prostate cancer. New source designs require full dosimetric analysis and calibration standardization before responsible clinical application. The results of one such experimental measurement and analysis are here reported for a new design of 103palladium source, model MED3633 (North American Scientific, Inc.). By AAMP Task Group #43 recommendations, the reference material for brachytherapy dosimetry is liquid water. The dose measurements were made using standard methods employing thermoluminescent dosimeters in a water equivalent plastic phantom. Precision machined bores in the phantom located dosimeters and source(s) in a reproducible 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 terms of parameters recommended by AAPM TG43. The dose-rate constant, lambda, was evaluated with reference to a 60 cobalt standard, accounting for response variation with isotope energy spectrum. The radial dose function, g(r), the anisotropy function, F(r, theta), the anisotropy factor, phi,un(r), and the point-source approximation anisotropy constant, phi(un), were derived from one- and two-dimensional dose distribution data measured in the phantom, accounting for finite dosimeter volume and with attention to interchip effects. The results are compared to TG43 and other existing data for 103Pd sources. The new source is comparable to the model 200 103Pd source design, demonstrating equivalent radial dose function, g(r). The dose surrounding a MED3633 source may be slightly more isotropic than for the model 200 source. The air-kerma strength of the MED3633 source used in this study was provided by the manufacturer and is traceable to the NIST 1999 standard. The evaluated dose-rate constant, lambda, with NIST traceable strength calibration is lower than that of the model 200 source, with that manufacturer's strength calibration.