A generic off‐axis energy correction for linac photon beam dosimetry

Cooperative clinical trial group protocols frequently require off‐axis point dose calculations. The Radiological Physics Center uses the calculative technique developed by Hanson et al. [Med. Phys. 7 , 145–146 (1980); 7 , 147–150 (1980)] to verify these calculations. In order to correct for off‐axis energy changes, this technique requires off‐axis half‐value layer data, HVL, as a function of off‐axis ray angle for the specific beam. This paper presents a formulism based on HVL measurements on a limited number of therapy beams, which allows the calculation of an off‐axis energy‐correction factor for any clinical photon beam created by a linear accelerator using conventional flattening filters.