[Changes of the dose at the surface in oblique incidence of high energy photon beams]

For oblique incident photon beams, the absorbed dose is markedly different from normal incident beams. Our aim was to study the changes of the dose at the surface and in the build-up region for oblique incident 4, 6, 10, 15, 25 MV photon beams angled 0 degree to 80 degrees for square fields ranging from 5 x 5 to 30 x 30 cm2. All doses were measured as a function of the depth along the beam axis and used to define an obliquity factor. For all energies investigated, we have studied the obliquity factor as a function of depth, field sizes, energy and angle of incidence. Our results show that the obliquity factor is highly dependent on these different parameters. In addition, by considering the equivalent squares of the entrance fields distorted by obliquity, an analytical method has been developed to predict the dose at the surface of high energy photon beams for various angles and field sizes.