ANISOTROPY FUNCTIONS FOR PD-103, I-125, AND IR-192 INTERSTITIAL BRACHYTHERAPY SOURCES

Anisotropy of dose distributions around Pd-103, I-125, and Ir-192 sources for interstitial brachytherapy was examined. Dose rates around I-125 models 6702 and 6711 and Ir-192 sources were measured using lithium fluoride thermoluminescence dosimeters (LiF TLDs) in a water-equivalent, solid phantom. From these measured data for I-125 and Ir-192 and the previously published measured data for Pd-103, isodose rate contours were determined using a bivariate interpolation and smooth surface fitting algorithm. The anisotropy functions, F(r,theta), as defined by the Interstitial Collaborative Working Group (ICWG) for each source, were determined. Also, 4pi-averaged anisotropy factors, phi(an)(r), for use in point source approximation, have been calculated at radial distances varying from 1-10 cm for Pd-103, I-125, and Ir-192 sources. The anisotropy factors had average values of 0.90, 0.93, 0.95, and 0.98 for Pd-103, I-125 model 6711, model 6702, and Ir-192, respectively. The anisotropy factors determined from dose measurements in phantom are observed to be closer to unity than from those determined previously from in-air measurements. This can be attributed to the smoothing of two-dimensional dose distributions due to the presence of more scattered photons in the phantom measurements compared to in-air measurements. Because in-phantom measurements simulate more closely the brachytherapy patient, data from these experiments are recommended for a more accurate determination of dose distributions around clinical brachytherapy implants. In this work, we present a complete set of source data for two-dimensional dosimetry following the ICWG formalism.