2-DIMENSIONAL SCATTER INTEGRATION METHOD FOR BRACHYTHERAPY DOSE CALCULATIONS IN 3D GEOMETRY

In brachytherapy clinical practice, applicator shielding and tissue he terogeneities are usually not explicitly taken into account. None of t he existing dose computational methods are able to reconcile accurate dose calculation in complex three-dimensional (3D) geometries with hig h efficiency and simplicity. We propose a new model that performs two- dimensional integration of the scattered dose component. The model cal culates the effective primary dose at the point of interest and estima tes the scatter dose as a superposition of the scatter contributions f rom pyramid-shaped minibeams. The approach generalizes a previous scat ter subtraction model designed to calculate the dose for axial points in simple cylindrically symmetric geometry by dividing the scattering volume into spatial regions coaxial with the source-to-measurement poi nt direction. To allow for azimuthal variation of the primary dose, th ese minibeams were divided into equally spaced azimuthally distributed pyramidal volumes. The model uses precalculated scatter-to-primary ra tios (SPRs) for collimated isotropic sources. Effective primary dose, which includes the radiation scattered in the source capsule, is used to achieve independence from the source structure. For realistic model s of the Ir-192 HDR and PDR sources, the algorithm agrees with Monte C arlo within 2.5% and for the I-125 type 6702 seed within 6%. The 2D sc atter integration (2DSI) model has the potential to estimate the dose behind high-density heterogeneities both accurately and efficiently. T he algorithm is much faster than Monte Carlo methods and predicts the dose around sources with different gamma-ray energies and differently shaped capsules with high accuracy.