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 cal culative technique developed by Hanson et al. [Med. Phys. 7, 145-146 ( 1980); 7, 147-150 (1980)] to verify these calculations. In order to co rrect for off-axis energy changes, this technique requires off-axis ha lf-value layer data, HVL, as a function of off-axis ray angle for the specific beam. This paper presents a formulism based on HVL measuremen ts on a limited number of therapy beams, which allows the calculation of an off-axis energy-correction factor for any clinical photon beam c reated by a linear accelerator using conventional flattening filters. (C) 1998 American Association of Physicists in Medicine.