Determination of the equivalent field size of asymmetric fields in photon beam therapy analytically based on physical theories

The aim of this study was to obtain relatively theoretical methods based on physical rules in interaction of radiation with matters which can be sensitively, rapidly and easily used in different radiotherapy centers.Subjects and In this study with the aid of the exponential attenuation rules of photon beams and inverse square distance rule, in 6MV and 18MV energy of 2100C Varian accelerator X-ray and 1.25MeV energy of Cobalt-60 γ-ray in Ahvaz Golestan Hospital and considering the scatter radiation of therapeutic field and irregular field to be equal, the side of equivalent square field were calculated. In this method, since the calculation was done taking into account the situation of the field, energy and position of the shield, a very good agreement with dose measurements showed (with error, less than 2%). Not all of these considerations are taken into account in previous methods. In addition, the calculations can be done on a computer for each therapeutic irregular field.