 Solution

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.