Past exam of the short options paper shared by Parts A and B of the physics course of the University of Oxford / 2019 / S10 / 3 / b

A localized paediatric intracranial tumour is a representative indication where proton therapy can reduce unnecessary absorbed dose to the developing brain and other normal tissue. The finite particle range can reduce exit and integral dose compared with many photon radiotherapy plans, which may reduce late effects and secondary cancer risk over a long expected survival. It remains necessary to maintain target coverage, account for changing anatomy and keep uncertain distal-edge relative biological effectiveness away from sensitive structures. Travel, cost and daily positioning are practical disadvantages. The relevant comparison is a robust individual treatment plan and clinical endpoint, rather than the label “proton” alone.^[ref]

A strongly moving lung target is a relative contraindication to a particular proton therapy plan if respiratory motion and changing air–tissue paths make robust target coverage and normal-tissue constraints unattainable. A changing water-equivalent path changes the particle range; pencil-beam scanning can also interact with respiratory motion. A photon radiotherapy plan may then offer more reliable coverage despite its greater exit dose. This is a conditional planning contraindication, not a blanket claim that lung cancer cannot be treated with protons: gating, breath-hold, rescanning and adaptive planning can change the decision. If a robust comparative plan shows no meaningful benefit, the additional cost and access burden also weigh against the proton plan.^[ref]