Dose buildup for obliquely incident photon beams.

For obliquely incident photon beams, the buildup of dose with depth is markedly different from normally incident beams. However, relatively little data on this topic exists for high-energy photon beams of energy greater than 6 MV. Measurements of dose in the buildup region were made using a plane-parallel ionization chamber in a polystyrene phantom with obliquely incident 6-, 10-, 18-, and 24-MV x-ray beams angled 0 degrees to 84 degrees. Buildup curves at these angles were plotted and from these an obliquity factor, defined as the ratio of ionization charge collected at a point for a particular angle of incidence to that collected at the same point at normal incidence, was determined. For each energy, the obliquity factor as a function of depth, field size, and source-chamber distance was studied. Results indicate that the obliquity factor is highly dependent on the beam energy, angle of incidence, the collimator opening, and the source-skin distance. A mathematical expression has been developed to predict the dose in the buildup region of high-energy photon beams for various angles of beam incidence, field size, and chamber distance.