USING THE FIELD EDGE CORRECTION (FEC) METHOD TO GENERATE ACCURATE POCRS AND OCRS FOR ASYMMETRIC FIELDS

Dose planning programs originally intended for use with symmetric fiel ds have been adapted for use with asymmetric fields. An accurate repre sentation of the change in primary beam quality with off-axis distance and depth is essential for accurate dose calculation and is usually r epresented in the computer as a primary radiation profile or primary o ff-center ratio (POCR). The original field edge correction (FEC) metho d described by Cadman [Med. Phys. 22, 457 (1995)] to determine POCRs h as been extended to allow accurate POCR values to be obtained to an of f-axis distance defined by the corners of the largest field, typically at an off-axis distance of 28.3 cm. This technique requires only rout ine symmetric field measurements including beam profiles, TMRs, and co llimator and phantom scatter factors. The POCRs obtained using the FEC technique were used to generate off-center ratios (OCRs) using the bo undary factor technique of Chui et al. [Med. Phys. 15, 92 (1988)]. Exc ellent agreement with measured values was obtained for cross-beam OCRs using a 10x10-cm(2) field defined by a single set of asymmetric jaws with a field center offset of 15 cm and for diagonal OCRs using a 20x2 0-cm(2) field with each pair of jaws in a half-blocked configuration. (C) 1996 American Association of Physicists in Medicine.