Convolution calculations of dose in the buildup regions for high energy photon beams obliquely incident

Abstract

The central dose received from high energy photon beams that are obliquely incident on an absorber is markedly different from that absorbed when such beams are normally incident at the surface. In this work, we calculate the dose differences using a convolution of energy deposition kernels in a phantom presenting an oblique entrance surface. The dose distributions were calculated with oblique incident 1.25, 3, and 6 MeV monoenergetic and polyenergetic photon beams angled from 0 degrees to 80 degrees. In order to study the dose variations with surface obliquity, we introduced an obliquity factor. For each energy, the obliquity factor was calculated as a function of depth and field size. We found that, ignoring the electron contamination from the air and from the treatment machine head, the influence of obliquity can be described in terms of upstream and downstream contribution of the electrons set in motion by the primary photons. In actual beams, especially for large field sizes, the electron contamination becomes significant and tends to reduce the influence of surface obliquity. Results indicate that the obliquity factor is highly dependent on the beam energy and depth, and are in good agreement with our experimental results measured for 10 and 25 MV x-ray beams. In this paper a theoretical explanation of these dose variations due to oblique incidence of the beam is presented.