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Monte Carlo modeling of the transverse‐axis dose distribution of the Model 200 103 Pd interstitial brachytherapy source
Author(s) -
Williamson Jeffrey F.
Publication year - 2000
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.598924
Subject(s) - monte carlo method , transverse plane , fluence , brachytherapy , calibration , materials science , physics , dosimetry , computational physics , inverse square law , optics , anisotropy , photon , kerma , electromagnetic shielding , nuclear medicine , mathematics , statistics , medicine , structural engineering , quantum mechanics , engineering , radiation therapy , laser , composite material , classical mechanics , gravitation
This study presents the first theoretical analysis of the absolute dose‐rate distribution about the Model 200103 Pd 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 the103 Pd ) plated onto the surfaces of right cylindrical graphite pellets contained within the seed. The dose‐rate constant, Λ, 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 Λ sensitive to the S Kmeasurement geometry. Fortunately, the WAFAC averages fluence over a sufficiently large aperture that the resultant Λ, 0.68±0.02 cGy⋅h −1 ⋅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 Λ value.