Anatomic changes in head and neck intensity-modulated proton therapy: Comparison between robust optimization and online adaptation

•A novel online adaptive strategy was compared to two robust optimization methods.•Daily CBCT images were used to introduce realistic setup and anatomical variations.•Robust optimization achieved adequate target coverage in up to 8/10 patients.•Online adaptation yielded adequate target coverage in all patients.•Online adaptation reduced the dose to most OARs compared to robust methods.