A Monte Carlo investigation of electron-beam applicator scatter.

An EGS4 Monte Carlo investigation into applicator scatter in clinical electron beams has been undertaken in order to establish the characteristics of electrons incident on the patient surface which have interacted with collimation systems. The applicator scattered component of an electron beam (including that from irregularly shaped cutouts) should be considered when modeling the electron phase space since it represents a component of the beam incident on the patient with widely varying characteristics to those of the primary beam. Scattering off an edge of applicator material is considered in terms of the types and characteristics of incident primary beam, the resulting interactions in the edge, and the fluence and energy characteristics of the emerging particles. Results indicate that the principal component to consider is scattered electrons due to the electron component of the primary beam, and that the fluence and energy characteristics of this component are dependent upon primary beam energy and the configuration of the applicator apertures.