Two-effective-source method for the calculation of in-air output at various source-to-detector distances in wedged fields

A simple algorithm was developed for calculation of the in-air output at va rious source-to-detector distances (SDDs) on the central axis for wedged fi elds. In the algorithm we dealt independently with two effective sources, o ne for head scatter and the other for wedge scatter. Varian 2100C with 18 a nd 8 MV photon beams was used to examine this algorithm. The effective sour ce position for head scatter for wedged fields was assumed to be the same a s that for open fields, and the effective source position for wedge scatter was assumed to be a certain distance upstream from the physical location o f the wedge. The shift of the effect:ive source for wedge scatter, MI, was found to be independent of field size. Moreover, we observed no systematic dependency of w on wedge angle or beam energy. One value, w=5.5 cm, provide d less than 1% difference in in-air outputs through the whole experimental range, i.e., 6x6 to 20x20 cm(2) field size (15x20 cm(2) for 60 degrees wedg e), 15 degrees-60 degrees wedge angle, 80-130 cm SDD, and both 18 and 8 MV photon beams. This algorithm can handle the case in which use of a tertiary collimator with an external wedge makes the field size for the determinati on of wedge scatter different from that for head scatter. In this case, wit hout the two-effective-source method, the maximum of 4.7% and 2.6% differen ce can be given by the inverse square method and one-effective-source metho d in a 45 degrees wedged field with 18 MV. Differences can be larger for th icker wedges. Enhanced dynamic wedge (EDW) fields were also examined. It wa s found that no second effective source is required for EDW fields. (C) 199 9 American Association of Physicists in Medicine.