A simplified method for the calculation of equivalent squares of irregular photon fields.

It is showed that the empirical scatter-radius function (Day function) used to generate the equivalent-field tables is a good approximation to the results of normalized scatter-air ratios extracted from the British Journal of Radiology (BJR) beam data for depths up to 10 cm. Accuracy can be improved by making the Day function depend on depth. Equivalent squares, determined by sector integration of the original or modified Day functions, are suitable for megavoltage photon-beam dose calculations in irregular as well as rectangular fields. We make further simplification for the functions to calculate equivalent squares by no use of scatter dose data that is often difficult to get for hospitals. The simplified method has the same accuracy compared to the original or modified Day functions.