= 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.
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