title: An equivalent square field formula for determining head scatter factors of rectangular fields.
abstract: A simple formula is derived for the calculation of an equivalent square field that gives the same head scatter factor as a given rectangular field. This formula is based strictly on the configuration of a medical linear accelerator treatment head. The geometric parameters used are the distances between the target and the top of each field-defining aperture. 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 require 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 format as the formula that was empirically obtained by Vadash and Bjärngard [Med. Phys. 20, 733-734 (1993)].
authors.affiliation: Department of Radiation Oncology, University of Florida College of Medicine, Gainesville 32610-0385, USA.
authors.forename: S
authors.initials: S
authors.lastname: Kim
authors.forename: T C
authors.initials: TC
authors.lastname: Zhu
authors.forename: J R
authors.initials: JR
authors.lastname: Palta
mesh_terms: D055592:Biophysical Phenomena; D001703:Biophysics; D006801:Humans; D008962:Models, Theoretical; D010315:Particle Accelerators / Q000295:instrumentation / Q000706:statistics & numerical data*; D011882:Radiotherapy, High-Energy / Q000295:instrumentation / Q000706:statistics & numerical data*; D012542:Scattering, Radiation