The validity of the approximation method "area/perimeter" as applied to 8 and 25-MV photon beams

The approximation method of area/perimeter for different portal sizes used in practice for finding the equivalent squares of a rectangular field has been tested on a 35-MV Clinac Linear Accelerator for the purpose of treatment planning. Data were obtained for 8 and 25-MV x-rays at depths of maximum build-up regions, and at 10 cm depth below the surface of a suitable water phantom. Maximum variation of dose calculation from the above method is less than 1.6% for routine clinical usage while less than 3.2% for extreme rectangular field sizes. Dose variation improves as the depth in phantom increases.