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 fields have been adapted for use with asymmetric fields. An accurate representation of the change in primary beam quality with off-axis distance and depth is essential for accurate dose calculation and is usually represented in the computer as a primary radiation profile or primary off-center ratio (POCR). The original field edge correction (FEC) method described by Cadman [Med. Phys. 22, 457 (1995)] to determine POCRs has been extended to allow accurate POCR values to be obtained to an off-axis distance defined by the corners of the largest field, typically at an off-axis distance of 28.3 cm. This technique requires only routine symmetric field measurements including beam profiles, TMRs, and collimator and phantom scatter factors. The POCRs obtained using the FEC technique were used to generate off-center ratios (OCRs) using the boundary factor technique of Chui et al. [Med. Phys. 15, 92 (1988)]. Excellent agreement with measured values was obtained for cross-beam OCRs using a 10 x 10-cm2 field defined by a single set of asymmetric jaws with a field center offset of 15 cm and for diagonal OCRs using a 20 x 20-cm2 field with each pair of jaws in a half-blocked configuration.
Cadman
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Physics Services, Saskatoon Cancer Centre, Saskatchewan, Canada.
D055592:Biophysical Phenomena; D001703:Biophysics; D006801:Humans; D008962:Models, Theoretical; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods* / Q000706:statistics & numerical data; D013679:Technology, Radiologic / Q000706:statistics & numerical data