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Tissue inhomogeneity correction for brachytherapy sources in a heterogeneous phantom with cylindrical symmetry
Author(s) -
Meigooni Ali S.,
Nath Ravinder
Publication year - 1992
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 - 10.1118/1.596894
Subject(s) - imaging phantom , monte carlo method , brachytherapy , materials science , photon , computational physics , polystyrene , dosimetry , homogeneous , physics , optics , nuclear medicine , nuclear magnetic resonance , mathematics , statistical physics , radiation therapy , medicine , statistics , polymer
In brachytherapy it is customary to perform dose calculations for an implant assuming that the tumor and surrounding tissues constitute a uniform, homogeneous medium equivalent to water. In this work, the validity of the above assumption is studied quantitatively for points along the transverse axis of 103 Pd, 125 I, and 241 Am brachytherapy sources, using measured and Monte Carlo calculated dose rates in homogeneous and heterogeneous media with cylindrical symmetry. The irradiation geometry chosen was a single source implanted in a Solid Water phantom which had a 1‐ or 2‐cm‐thick cylindrical Solid Water shell replaced by a polystyrene shell. The Monte Carlo simulations were performed using the integrated tiger series CYLTRAN Code. Experimental data were obtained for the same geometry to test the validity of the Monte Carlo calculations for a heterogeneous phantom. Measured dose rates just beyond a 2‐cm‐thick polystyrene heterogeneity were observed to be greater than those in a homogeneous Solid Water phantom by about 130%, 55%, and 10% for 103 Pd, 125 I, and 241 Am, respectively. Thus the effect of a relatively small polystyrene heterogeneity in Solid Water can be substantial for lower energy photons. This perturbation of dose was found to increase steeply with decreasing energy and increasing size (thickness) of inhomogeneity. A simple dose calculation formalism has been developed to predict dose rate in a heterogeneous phantom with cylindrical symmetry, which uses as input the radial dose functions of the uniform media comprising the heterogeneous phantom. Dose rate predictions using this formalism are in reasonable agreement with the experimental data and the Monte Carlo calculated values. Also, this formalism predicts no inhomogeneity effect for 192 Ir sources in the geometrical configuration studied in this work.