Tissue-maximum ratios for high-energy x rays

The tissue-maximum ratios for high-energy betatron x rays, measured at target-axis distances of 90, 110, and 130 cm, exhibit a strong dependence on the target-axis distance in the buildup region as well as a significant dependence in the exponential region. This dependence and the field-size dependence of the tissue-maximum ratios at a fixed target-axis distance are studied with the use of phantom and collimator scatter factors. The depth and field-size dependence of these scatter factors are determined experimentally. The measurements indicate that the collimator scatter factor is dependent on both the collimator opening and the targetaxis distance. These components are quantitated separately and are associated primarily with high-energy electron scatter from the collimating system. An equation involving these components of the collimator scatter factor is derived for the conversion of tissue-maximum ratios from one target-axis distance to another. With the use of this equation, the tissue-maximum ratios measured for one target-axis distance can be used to calculate within 1% those at other target-axis distances.