A new approach to CT pixel‐based photon dose calculations in heterogeneous media

Abstract

The effects of small cavities on dose in water and the dose in a homogeneous nonunit density medium illustrate that inhomogeneities do not act independently in photon dose perturbation, and serve as 2 constraints which should be satisfied by approximate methods of computed tomography (CT) pixel-based dose calculations. Current methods at best satisfy only 1 of the 2 constraints and show inadequacies in some intermediate geometries. An approximate method was developed that satisfies both these constraints and treats much of the synergistic effect of multiple inhomogeneities correctly. The method calculates primary and 1st-scatter doses by 1st-order ray tracing with the 1st-scatter contribution augmented by a component of 2nd scatter than behaves like 1st scatter. Multiple-scatter dose perturbation values extracted from small cavity experiments are used in a function which approximates the small residual multiple-scatter dose. For a wide range of geometries tested, our method agrees very well with measurements. The average deviation is less than 2% with a maximum of 3%. In comparison, calculations based on existing methods can have errors larger than 10%. [Because of the steepness of the radiation dose-response curves for tumors and normal tissues, precise dose delivery is imperative in radiation therapy.].