The use of tissue-air ratio with high-energy photon fields

Conventional dosimetry for photon beams below 5 MV uses tissue-air ratio (TAR). This present work shows that the conventional objections to the use of TAR-based dosimetry for higher-energy beams, including the inability to measure in-air dose, failure of lateral electronic equilibrium in the build-up cap for small fields, and Compton scatter from the build-up cap contributing to the in-air dose, all can be overcome. In addition, this work describes minor modifications of commercial scanning systems to measure TAR directly. This dissertation shows that a tissue-maximum ratio system propagates errors or degenerate into the TAR system. As with lower energy radio therapy beams, TAR forms the dosimetery system of choice for higher energy beams. While this work shows that the tissue-air ratio proves sufficient for all simple dose calculations without the need for scatter-air ratios, for complex situations a differential tissue-air ratio has been derived to describe the dose contribution from one volume element to another