Output factors and scatter ratios for radiotherapy units

Abstract

Variables such as scatter-air ratios and scatter functions which describe the radiation scattered to a location of interest within a patient are defined with the implicit assumption that only primary radiation impinges on the patient. That is, radiation scattered from the source and collimator are not considered in the definitions, and the dose rate in air is assumed to remain constant with changes in field size. In actual practice, significant scattered radiation impinges upon the patient and dose rate in air varies with field size in a manner described by the output factor. The output factor should be incorporated into definitions of variables which describe the scattered radiation component to the total dose delivered to a location of interest within the patient. As examples, output factors are incorporated into variables describing scattered radiation for both 60Co .gamma.-rays and megavoltage X-rays. Since scatter ratios are essential for dose computations for irregularly shaped fields, an accurate definition of these variables is important. Radiation scattered from the source and collimator is mixed with primary radiation in the X- or .gamma.-ray beam impinging on the patient. This scattered radiation should be included in the scatter ratios.