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How is photon head scatter in a linear accelerator related to the concept of a virtual source?

The virtual source in a photon linear accelerator is a familiar concept which arises from head scatter, and which accounts for small but detectable deviations of photon energy fluence from the inverse-square law. The authors present a simple method for calculating the position of the virtual source using the geometry of the flattening filter, which acts as the main photon scatterer. The authors also show that, for unwedged beams, the position of the virtual source is relatively insensitive to collimator size. This means that the same head-scatter factor which is measured at the isocentre can also be used at other distances from the source. However, wedges may scatter so much radiation that the position of the virtual source depends upon collimator size.