Day's integration of scatter dose with an analytical expression.

Analytical formulae have been developed to describe the scatter dose on the central axis of rectangular fields as a function of depth and field size for 60Co gamma radiation and 4 and 8 MV x rays. The basic relationship expresses the scatter-to-primary dose ratio as a function of the area-to-perimeter ratio A/P, with depth and energy dependent parameters for which analytical expressions are given. The accuracy of the total dose on and off the central axis, calculated with these formulae, is compared to that resulting from the use of Day's method with tabulated values. The differences are generally less than 1% of the total dose on the central axis.