AN EQUIVALENT SQUARE FIELD FORMULA FOR DETERMINING HEAD SCATTER FACTORS OF RECTANGULAR FIELDS

A simple formula is derived for the calculation of an equivalent squar e field that gives the same head scatter factor as a given rectangular held. This formula is based strictly on the configuration of a medica l linear accelerator treatment head. The geometric parameters used are the distances between the target and the top of each field-defining a perture. The formula accounts for both the effect of field elongation and the collimator exchange effect. This method predicts the output to within 1% accuracy for both open and wedged fields and does not requi re any new measured data other than the field size dependence of head scatter for a range of square field sizes. Interestingly, the formula we derived has the same formal as the formula that was empirically obt ained by Vadash and Bjarngard. (C) 1997 American Association of Physic ists in Medicine. Medicine.