Dose perturbation by air cavities in megavoltage photon beams: implications for cavity surface doses.

Gas-filled cavities in the body are known to perturb megavoltage photon beams in radiation therapy. The bowel represents one such cavity where cavity dimensions can vary throughout a treatment course. This has implications for doses to the surface of the bowel. Doses to the first 0.1 mm and 1.0 mm of cavity surfaces were calculated for 6 MV and 18 MV photon beams for a range of cavity sizes. It is found that relatively minor variations in cavity surface doses result over the typical range of rectal cavity size (2-3 cm). However, the variations in surface dose should be considered whenever bowel reactions must be considered, or when the target cells of interest line the bowel itself.