Proton Beam Therapy is probably the most interesting Medical Physics topic I can think of, maybe comparing it to different radiotherapy methods, the feasibility of it having a large impact upon how cancers are dealt with in general and whether or not the NHS should implement it for most cancer patients currently undergoing radiotherapy. If you're in the London area, UCL Science Centre is doing a public lecture on the topic in a few weeks time which could prove to be useful for your EPQ.