Empirical dosimetric characterization of model I125-SL (125)Iodine brachytherapy source in phantom

Low-energy photon emitting radionuclides encapsulated for a permanent impla nt 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 analy sis are reported here for a new design of I-125 source, model I125-SL. Dose measurements were made using standard methods employing thermoluminscent d osimeters in a water equivalent plastic phantom, in accord with the AAPM Ta sk Group #43 recommendation of liquid water reference material. 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 10 cm. The data were analyzed in te rms of parameters recommended by AAPM TG43. The dose-rate constant, Lambda, was evaluated by two methods, the first with reference to a (60)Cobalt sta ndard, accounting for response variation with photon energy spectrum. Secon d, the dose-rate constant was determined with reference to phantom measurem ents using NIST traceable calibrated model 6702 and 6711 sources. The radia l dose function, g(r), the anisotropy function, F(r,theta), the anisotropy factor, phi (an)(r), and the point-source approximation anisotropy constant , <(<phi>)over bar>(an), were derived from one- and two-dimensional dose di stribution data measured in the phantom, accounting for finite dosimeter vo lume and with attention to interchip effects. The results are compared to T G43 and other existing data for I-125 sources. The new source is comparable to the model 6711 source design. (C) 2000 American Association of Physicis ts in Medicine. [S0094-2405(00)00112-7].