Calculation of dose in asymmetric photon fields.

A method is introduced to calculate monitor units to points off axis. Extensive data are presented comparing this method with measured values of dose per monitor unit on the central ray of asymmetric fields produced by a variety of linear accelerators. The technique demonstrates improvement over existing methods that use large-field profile data. The method is found to be both simple and accurate: Agreement within +/- 2% is obtained using parameters readily available within the clinic.