A systematic review of health economic evaluations of proton beam therapy for adult cancer: Appraising methodology and quality.

•The cost-effectiveness of PBT for adult cancers is subject to considerable uncertainty.•Cost-utility analysis is the gold standard for assessing value, but quality matters.•Our review found studies lacked external validation of model outcomes.•When assessed against standard checklists, studies fell short.•Improving transparency and validation will improve credibility of results.