Off-axis output factors for 6MV and 18MV photons

A series of in-air measurements showed that collimator scatter (Sc) did not change significantly for 6MV photons when the centre of the field was moved away from the central axis. This result enabled a model to be developed for the off-axis Effective Output Factor (EOF) which was then verified for 6MV and 18MV photons on a Varian 2100c accelerator and for 6MV photons on a Varian 600c accelerator. Thus off-axis output factors may be predicted, for a range of rectangular asymmetric fields, using only the Primary Off-Centre Ratio (POCR) in air and the on-axis output factor. Depth doses were also investigated off-axis and found to have no clinically significant differences compared with on-axis depth doses, for depths less than 7.5 cm for 6MV and 12.5 cm for 18MV photons. The model is simple to implement and avoids the need for a measurement for each patient, thus saving accelerator time.