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 40×40 cm2. The wedge factor for 60°, 45°, 30°, and 15° 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 5×5 to 40×40 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° wedge, all wedged beams were softer than the open beams, for all field sizes. At 10 MV, wedged fields of size less than 20×20 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.