The use of an universal wedge for asymmetric fields

Beam collimators on newer linear accelerators may be collimated asymmetric to the central axis. The asymmetric beam has a non-flat profile adjusted to yield fields whose half widths are not symmetric about the central axis. While some treatment planning systems modify their programs to mimic the nonuniformity, ideally it is preferred to have a flat profile under the open beam. We have developed a universal wedge that can be used to flatten the field for a variety of jaw sizes and positions and energies for the Varian 2100C. The wedge flattens the field to +/- 3% over 80% of the field.