Scattered photons produced by beam-modifying filters.

When a beam-modifying filter such as a wedge or a compensator is placed in an x-ray beam, scattered photons are generated in the filter material. The magnitude of the dose contribution from these photons for a 4-MV x-ray beam was measured. At a distance of 30 cm from the filter, a copper sheet of 1-cm thickness produced a dose contribution on the centerline of about 6% of the transmitted primary dose in a 20 X 20 cm2 field. At the edges of the beam, this contribution was only about one-half that on the centerline. The presence of these scattered photons leads to only minor dose errors in clinical applications if simple procedures are followed to account for their contribution.