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] 