A model for electron-beam applicator scatter.

Applicators (or cones), used in conjunction with patient specific cutouts in electron-beam radiotherapy, may interact with the primary electron beam to produce a secondary beam component (applicator scatter). This component affects machine output as well as the shape of resulting dose distributions. A model has been developed to simulate this scatter component for applicators consisting of trimming plates of arbitrary shape. This model involves sampling established kernels of scatter from edge elements of appropriate materials, obtained through Monte Carlo simulations. The result of the model is a phase space (position, direction, energy, charge, weighting) of applicator scattered particles which can be incorporated into a further Monte Carlo simulation, or as input into another advanced treatment planning algorithm. This model is evaluated by comparison of measured profiles and applicator scatter component depth dose curves with Monte Carlo simulations using simulated phase-space data as input. Results are very consistent and reveal information on the angular and spatial variation characteristics of this beam component. The results obtained verify the developed model as an accurate predictor of the characteristics of applicator scattered particles.