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

Photon radiotherapy uses indirectly ionizing photons. Their Compton scattering, photoelectric effect and, at higher energy, pair production create charged particles that deposit energy. A megavoltage beam has a dose-build-up region followed by a gradual decline and substantial exit dose. Multiple shaped beams or intensity-modulated radiotherapy distribute this dose around a target.

Proton therapy uses directly ionizing charged particles with a finite particle range and a Bragg peak. Tuning beam energy controls depth and scanning controls lateral delivery; multiple energies cover a finite target thickness. Reduced distal and integral normal-tissue dose is a potential advantage. Limitations include range uncertainty, Multiple Coulomb scattering, nuclear secondary production, motion-dependent delivery, and a variable relative biological effectiveness, particularly near the distal edge. A physical dose advantage must be assessed against target coverage and clinically relevant toxicity; it does not by itself establish superior tumour control.

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]

A valid comparison needs comparable disease stage, risk, systemic treatments and dose prescriptions; otherwise selection bias and confounding can masquerade as treatment benefit. Referral to specialist centres can select younger, wealthier or more complex patients. Randomized controlled trials reduce such biases but can face limited recruitment, patient preference, crossover and lack of clinical equipoise.

Both arms need expert contemporary planning and comparable quality assurance. Centre experience, changing delivery technology and an obsolete comparison arm can distort results. The chosen endpoint must match the proposed benefit: equal tumour control with lower late toxicity may be valuable even without a survival difference. Statistical power, competing mortality, multiple comparisons and long follow-up are crucial for rare late effects, particularly second cancers. Blinding of treatment delivery is difficult, so independent endpoint adjudication and predefined analyses help. Economic analyses must state their perspective and include access and follow-up costs.