Monte Carlo modeling of the transverse-axis dose distribution of the Model 200 103Pd interstitial brachytherapy source

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Abstract
This study presents the first theoretical analysis of the absolute dose-rate distribution about the Model 200 103Pd 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 source as a function of thickness of the Pd metal coating (containing the 103Pd) plated onto the surfaces of right cylindrical graphite pellets contained within the seed. The dose-rate constant, A, was realistically estimated by simulating the wide-angle, free-air chamber (WAFAC) calibration geometry. The WAFAC is the experimental realization of NIST's (National Institute of Standards and Technology) recently implemented primary standard of air-kerma strength (S(K)). Our results show that polar angle- and distance-dependent oblique filtration and shielding effects induce significant and unexpected photon fluence anisotropy near the transverse-axis and inverse square law deviations at typical calibration distances. Any source consisting of radioactivity deposited on a highly attenuating surface with sharp edges may exhibit such effects. In the case of the Model 200 seed, the Pd metal thickness does not significantly influence the relative dose distribution in water at distances less than 5 cm, but does make A sensitive to the S(K) measurement geometry. Fortunately, the WAFAC averages fluence over a sufficiently large aperture that the resultant A, 0.68 +/- 0.02 cGy x h(-1) x U(-1), is almost independent of Pd metal layer thickness and in close agreement with recent measurements and calculations. This value is 20% higher than that of the renormalized Task Group 43 A value.
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