ANALYTICAL APPROACH TO HETEROGENEITY CORRECTION FACTOR CALCULATION FOR BRACHYTHERAPY

In brachytherapy treatment planning, the effects of tissue and applica tor heterogeneities are commonly neglected due to lack of accurate, ge neral, and fast three-dimensional (3D) dose-computational algorithms. A novel approach, based on analytical calculation of scattered photon fluxes inside and around a disk-shaped heterogeneity, has been develop ed for use in the three-dimensional scatter-subtraction algorithm. Spe cifically, our model predicts the central-ray dose distribution for a collimated photon isotropic source or brachytherapy ''minibeam'' in th e presence of a slab of heterogeneous material. The model accounts for the lateral dimensions, location, composition, density, and thickness of the heterogeneity using precalculated scatter-to-primary ratios (S PRs) for the corresponding homogeneous problem. The model is applicabl e to the entire brachytherapy energy range (25 to 662 keV) and to a br oad range of materials having atomic numbers of 13 to 82, densities of 2.7 g.cm(-3) (Al) to 21.45 g.cm(-3) (Pt) and thicknesses up to I mean free path. For this range of heterogeneous materials, the heterogenei ty correction factors (HCFs) vary from 0.09 to 0.75. The model underes timates HCF when multiple scattering prevails and overestimates HCF wh en absorption dominates. However, the analytic model agrees with Monte Carlo photon transport (MCPT) benchmark calculations within 1.8% to 1 0% for I-125, Yb-169, Ir-192, and Cs-137 for a wide variety of materia ls, with the exception of Ag. For I-125 Shielded by Ag, where the mean discrepancy can exceed 25%, the error is due to K-edge characteristic x rays originating within the heterogeneity. The proposed approach pr ovides reductions in CPU time required of 5x10(4)-10(5) and 100 in com parison with direct MCPT simulation and 1D numerical integration, resp ectively. The limitations of model applicability, as determined by the physical properties of heterogeneity material and accuracy required, are also discussed. (C) 1998 American Association of Physicists in Med icine.