Protons represent a logical step forward as a modality for radiotherapy because it is possible to concentrate dose in the tumor region and minimize dose to normal tissue. The pencil beam emerging from a cyclotron or synchrotron needs to be expanded to a larger size to cover tumors of realistic size. Ideally, this should be accomplished by scanning the pencil beam using magnetic fields, but this is technically challenging. The simpler method is to use passive modulation, the downside of which is that neutrons are produced in the scattering foil, which results in a total body dose to the patient. Neutrons are highly effective at inducing second cancers and so, for the full potential of protons to be realized, spot scanning is essential.

Key words: Protons; Neutrons; Cancer; Scanning beams; Passive modulation.