Output factors for squared, rectangular and elongated photon fi elds of linear accelerators: A supplement to Stirling’ formula

Abstract

For dose calculation in radiation therapy with photons a variety of basic beam data are required, such as percentage depth doses, dose profi les, and output factors (OF) for open as well as for wedged beams. In contrast to profi le measurements, output factors have to be measured for all squared, rectangular and also for extremely elongated fi eld sizes. To realize smooth output factor functions, at least 100 different fi eld sizes with small increments in length and width have to be measured. A simple modifi cation of the Stirling’ formula, taking into consideration the collimator exchange effect, allows an accurate prediction of the output factors for all fi eld sizes, based on the experimental data survey for the minimum, the maximum and the reference fi eld size (10 x 10 cm2) only. The ratio of the calculated OF and the measured one’s stays within 1.0025 ± 0.009, even for elongated and wedged fi eld sizes. All major vendors of medical linear accelerators have been included in this study.