Dynamic universal wedge.

A computer-controlled equivalent of the universal wedge was designed by moving one collimator with step-wise constant velocity to produce the same primary-dose profile as a 20x20-cm2 conventional 58-deg wedge. It was used for smaller fields without changing the profile and combined with open beams to alter the wedge angle. Output factors in air and wedge factors in air and water were determined experimentally for the dynamic wedge and compared to predictions based on the assumption that the primary dose is proportional to the number of monitor units that the point of interest is in the open portion of the beam. This model was found to be accurate within about 2% and the deviations caused by head-scatter changes and collimator transmission after the moving collimator has passed. Measurements of the wedge factor in water indicated that the scatter factors for large wedged beams slightly exceeded those for open beams.