DOSIMETRY OF LARGE WEDGED HIGH-ENERGY PHOTON BEAMS

The dependence of the wedge factor and central axis depth dose on field size was evaluated for 6-, 10-, and 24-MV wedged photon beams for field sizes up to 40 X 40 cm2. The wedge factor for 60-degrees, 45-degrees, 30-degrees, and 15-degrees wedges in a 24-MV beam was found to vary by as much as 25 %, 12%, 9%, and 5%, respectively, over a field size range of 5 X 5 to 40 x 40 cm2. For 10 and 6 MV wedged beams, the wedge factors varied by up to 17% and 15%, respectively, over the same field size range. The depth dose curves for the wedged beams differed significantly from the open beam profiles. At 6 MV, the wedges caused beam hardening while at 24 MV, with the exception of the 15-degrees wedge, all wedged beams were softer than the open beams, for all field sizes. At 10 MV, wedged fields of size less than 20 X 20 cm2 were hardened relative to the open beam, whereas larger wedged fields had depth dose values within +/- 1% of the 10-MV open-beam depth dose data. Accurate treatment planning for large wedged fields and high-energy photon beams thus requires the use of wedged beam depth dose curves and field size specific wedge factors. It was established that an equivalent square field for a rectangular wedged field can be determined using the standard open beam formulation. The largest difference between the wedge factor for a rectangular beam and its equivalent square beam was 2.5% and occurred for 24-MV elongated fields. The depth dose values for a rectangular wedged field and its equivalent square beam were within +/- 1% at all energies studied.