Scattered photons from wedges in high‐energy x‐ray beams

The presence of a wedge increases the fraction of “head‐scattered” photons in a high‐energy x‐ray beam. We have compared internal and external wedges for x‐ray beam energies between 6 and 25 MV by determining their SPR w , i.e., the ratio of the dose contribution from photons scattered by the wedge and photons either coming directly from the target or scattered by other structures, including the flattening filter. Marked differences were observed. First, SPR w for the thickest external wedge (60°) was 1.8% at a field size of 10×10 cm 2 and mildly dependent on the photon energy, while SPR w for internal wedges for this field size varied between 4.4% and 5.4% depending mostly on the location and size of the wedge and marginally on the photon energy. Second, the variation of SPR w with the collimator setting c × c was different for the internal and external wedges. SPR w for the internal wedge approached a limiting value at large c and could be fitted with an error function, while SPR w for the external wedge increased quadratically with c . As a result, the difference in SPR w ( c ) for internal and external wedges is reduced for large fields.