A generalized extension to the Hounsell-Wilkinson head scatter model.

Hounsell and Wilkinson (1997 Phys. Med. Biol. 42 1737-49) have demonstrated that the concept of a scatter-plane source can accurately describe head scatter in irregularly shaped fields produced by the MLC of an Elekta linear accelerator. In these linacs, the solid jaw is below the MLC set and plays no part on defining the view of the main source of head scatter, the flattening filter. A more complicated situation arises for linear accelerator head designs which incorporate a solid upper jaw pair with the MLC jaws further from the source. Application of the Hounsell and Wilkinson technique to these accelerator designs does not achieve the accuracy in the Elekta application. We present a generalized extension to Hounsell and Wilkinson's model and present results for typical treatment field shapes on a Siemens Primus linear accelerator.