A two-source model for the calculation of relative output factors (ROF) for
clinical applications of electron beams has been developed. The model cons
ists of (1) an effective extended source above the final field-defining ape
rture (cutout) plane and (2) a source due to scattering from the aperture.
Calculations are based on Fermi-Eyges theory and a pencil beam algorithm wi
th parameters determined independently for each major scattering component.
The model predicts a modified inverse square law for determining the dose
rate for the electron beams. It also generalizes the "square-root method" a
nd "one-dimensional method" that are often used clinically for ROF calculat
ions. A computer program based on the model has been developed to calculate
ROF for irregular fields. The predictions of ROF values have been compared
with measurements on a Varian CLINAC 2100C/D accelerator for different cut
out size, energies, applicators, and SSDs for square fields, rectangular fi
elds, circular fields, and irregular fields. The agreement between predicti
on and measurement of the ROF for these wide range of conditions is general
ly within 1% for energies from 6 to 20 MeV. This two-source model can be us
ed for clinical applications and it requires a minimal set of measured inpu
t data. (C) 2001 American Association of Physicists in Medicine.