= Solution
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.<Respiratory motion management for particle therapy by AAPM>[Report 290, dose uncertainty from respiratory motion and need for site-specific planning studies]{ref}
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