A less empirical method of representing megavoltage beams for use in rapid radiotherapy dose calculations.

A large number of initial measurements are needed for accurate computation of radiotherapy dose distributions directly from stored data on the dose distribution of single radiation beams. Because of this, interest has been shown in alternative methods which use empirical formulae to represent the dose distributions of single beams. These can be easier to implement but are less directly related to measured quantities and may be of limited validity. A method of representing radiotherapy beams is proposed which closely approximates their radiation physics, in order to calculate the dose at any point by a simple algorithm directly from a small number of initial dose measurements. The method is of wide validity, probably including all megavoltage radiations for which Compton scattering is the dominant interaction in tissue. The accuracy of the method is demonstrated by application to an 8 MV linear accelerator and a cobalt 60 machine.