Dosimetric characterization of a new /sup 125/Iodine brachytherapy source, model I125-SL

Abstract

Low-energy gamma emitting isotopes encapsulated for permanent implant are routinely applied in prostate cancer brachytherapy. Before clinical use, a new source design requires full dosimetric analysis and calibration standardization. The results of one such experimental measurement and analysis are here reported for a new design of /sup 125/Iodine source, model I125-SL (Mills Biopharmaceutical Inc.). Dose measurements were made using standard methods employing thermoluminescent dosimeters in a water equivalent plastic phantom, in accord with the AAMP Task Group 43 recommendation of liquid water reference material. 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 10 cm. The data were analyzed in terms of parameters recommended by AAPM TG43. The dose-rate constant, /spl Lambda/, was evaluated by two methods, the first with reference to a /sup 60/Cobalt standard, accounting for response variation with isotope energy spectrum. Second, the dose-rate constant was determined with reference to phantom measurements using calibrated model 6702 and 6711 sources. The radial dose function, g(r), the anisotropy function, F(r,/spl theta/), the anisotropy factor, /spl phi//sub an/(r), and the point-source approximation anisotropy constant, /spl phi/~/sub an/, were derived from one- and two-dimensional dose distribution data measured in the phantom, accounting for finite dosimeter volume and with attention to inter-chip effects. The results are compared to TG43 data for existing /sup 125/I sources. The new source is comparable to the model 6711 source design.