Central axis tissue-air ratios (TAR) for 4 MV x rays of a large number of square, rectangular and irregular fields defined by a multi-split collimator for conformation therapy were obtained experimentally up to a depth of 25 cm. Families of TAR curves as a function of area at depth, divided by perimeter (A/P) for fixed values of depth were plotted first using the data for square fields. These were then compared with data for irregular fields. When the equivalent perimeter of the field Pe was used, rather than the real perimeter P, the A/P method for determining TAR was found to be applicable within a 2% error up to a 15 cm depth for uneven fields shaped with the multi-split collimator.
