Use of Monte Carlo software to aid design of a proton therapy nozzle

A second proton therapy nozzle is being developed at iThemba LABS to irradiate lesions in the body, thus complementing an existing facility for head and neck treatments. A passive scattering system is being developed, the complexity of which necessitates Monte Carlo simulations. We have used MCNPX to set the apertures and spacing of collimators, to model dose distributions in water, to check and modify beam scattering and energy modulating components, and to check radiation shields. The comprehensive shielding model was adapted for other problems by reducing the types of particles transported, limiting the extent and complexity of the geometry, and where possible killing particles by setting their importance to zero. Our results appear to indicate that the Rossi and Greisen description of multiple Coulomb scattering as used in MCNPX predicts high-Z, large angle scattering acceptably well for modeling proton therapy nozzles. MCNPX is easy to learn and implement, but has disadvantages when used to model therapy nozzles: (1) it does not yet offer a true capability to model electromagnetic interactions, (2) it cannot model moving components, and (3) it uses energy rather than range cut-offs for particles. Hence a GEANT4 model of the new nozzle is also being implemented.