Monte Carlo modeling of the transverse-axis dose distribution of the Model200 Pd-103 interstitial brachytherapy source

This study presents the first theoretical analysis of the absolute dose-rat e distribution about the Model 200 Pd-103 interstitial brachytherapy source . Monte Carlo photon-transport (MCPT) simulation techniques have been used to evaluate the transverse-axis dose-rate distribution of the Model 200 sou rce as a function of thickness of the Pd metal coating (containing the Pd-1 03) plated onto the surfaces of right cylindrical graphite pellets containe d within the seed. The dose-rate constant, Lambda, was realistically estima ted by simulating the wide-angle, free-air chamber (WAFAC) calibration geom etry. The WAFAC is the experimental realization of NIST's (National Institu te of Standards and Technology) recently implemented primary standard of ai r-kerma strength (S-K). Our results show that polar angle- and distance-dep endent oblique filtration and shielding effects induce significant and unex pected photon fluence anisotropy near the transverse-axis and inverse squar e law deviations at typical calibration distances. Any source consisting of radioactivity deposited on a highly attenuating surface with sharp edges m ay exhibit such effects. In the case of the Model 200 seed, the Pd metal th ickness does not significantly influence the relative dose distribution in water at distances less than 5 cm, but does make Lambda sensitive to the S- K measurement geometry. Fortunately, the WAFAC averages fluence over a suff iciently large aperture that the resultant Lambda, 0.68 +/-0.02 cGy.h(-1)U( -1), is almost independent of Pd metal layer thickness and in close agreeme nt with recent measurements and calculations. This value is 20% higher than that of the renormalized Task Group 43 Lambda value. (C) 2000 American Ass ociation of Physicists in Medicine.