Equation for scatter-air ratios for megavoltage radiotherapy calculations.

Scatter-air ratios are tabulated quantities which are based on measurement and which are used to calculate the dose to points in the body of a patient exposed to radiation fields of arbitrary shape. For computing purposes it is convenient to represent these quantities by an equation. We have found a nine-parameter empirical function which expresses scatter-air ratio data for 60Co gamma rays and for 4- and 6-MV x rays. To test the equation it was used to calculate tissue-air ratios for a range of rectangular fields. The results are in good agreement with published values.