Prediction of electron beam output factors

Abstract

A method to predict square and rectangular field output factors from the measurement of selected fields of electron beams on the Therac 20 Saturne was developed. A 2 parameter fit of the square field output factor data, based on the functional dependence as predicted by a pencil beam calculational model, proved clinically acceptable [in human radiotherapy]. The pencil beam distributions are given by the Fermi-Eyges theory of multiple Coulomb scattering. For a rectangular field, the output factor can be calculated from the square root of the product of the 2 square field output factors with sides equal to those of the rectangular field. If, however, there is a significant asymmetry between the X and Y collimator systems, then rectangular field output factors should be predicted from the product of the X and Y 1-dimensional output factors. One-dimensional output factors are defined as output factors of rectangular fields where 1 side remains constant and equal to the side of the square reference field. Measured data indicate either of the 2 methods of determining rectangular field output factors to be clinically acceptable for the Therac 20, the use of 1-dimensional output factors demonstrating greater accuracy. Data show agreement to within .apprx. 1.5% at electron energies of 6, 9, 13 and 17 MeV.