Calculation of the off-axis in-phantom scatter functions

Abstract

Semi-empirical algorithms used for dose calculation in high energy photon beams are usually based in central axis data. The applicability of them to points off axis has been assessed only for old calculation techniques, such those using tissue air ratio functions. Recently, a new formulation based on phantom scatter quantities i.e. the [D/Ψ]rel formalism, has been introduced for use in central axis and homogeneous media. The extension of this calculation methodology for points off axis has to deal mainly with spectral changes that must to be accounted for. In this work, a new multiplicative function P(d,r) is introduced to account for variations in the phantom scatter quantities, which depends on the phantom depth (d) and on the off-axis position (r). This function was calculated from measurements of dose profiles (off-axis profiles in water) and in-air profiles corresponding to photon beams of 6 and 15 MV. Additionally, the function [D/Ψ]rel was determined experimentally and compared with the theoretical prediction, with an agreement better than 2% in any case. This difference could be attributed to experimental uncertainties and is within the clinical tolerances for dose calculation.