Physical characterization of 45-MV photon beams for use in treatment planning.

Central-axis percentage depth doses and tissue-maximum ratios (TMR) for 45-MV photon beams from a betatron have been measured in water. Also the influence of field size and collimator scatter on the dose in the buildup region have been investigated. The maximum dose for TMR has been shown to occur at a point about 2 cm deeper than the maximum dose for percentage depth dose. This difference is significant in the characterization of the photon beam at this high energy. The measured physical data have been computerized for use in routine treatment planning. Computer-gererated beams have been found to be in close agreement with measured isodose curves. Computer-generated isodose distributions for typical clinical irradiation techniques have been verified using RP/V film in an Alderson phantom.