Dose calculations of irregularly shaped fields can be made by the Clarkson technique, which, however requires considerable time and is thus not practical. We investigated a simple approximation method for determining field factors (F_A) and tissue-peak ratios (TPRs) for irregularly shaped fields. By this method, we approximated scatter dose by the ratio of area for an irregularly shaped field to that for the overall field (without blocking). Maximum error of equivalent square fields as determined by this method for irregularly shaped fields was-1.3% for field factors, +2.1% for TPRs and +1.4% for the F_A×TPRs.
