Description of the scatter component in photon-beam data

Abstract

Kerma scatter-to-primary ratios (SPR) were calculated, using a Monte Carlo code, for water irradiated by photon beams (200 keV monoenergetic, ⁶⁰Co gamma , 4 and 15 MV X-rays). The SPR at depth d on the centreline of a circular field of radius r can be described by SPR=aZ+bZ², with Z=rd/(r+d). The proportionality constant a is determined by the linear attenuation coefficient mu and the ratio r/d. The coefficient b for the second-order term depends on r/d and energy, the latter by being proportional to mu ³. This relation between SPR and Z can facilitate analysis of beam data in terms of the contributions from primary and scattered photons, in particular for small depths and small fields where measurements are difficult. This is illustrated by examining measured TAR data for ⁶⁰Co gamma beams. While the relation between SPR and Z is not fully explained, it is demonstrated qualitatively that SPR approximately=aZ if only first-order scatter is to be accounted for.