PDCD1 and cancer: PBT combined with PD-1/PD-L1 inhibitors was considered safe and produced no unexpected adverse events.[41] In a single-arm, prospective phase II clinical trial of recurrent or metastatic head and neck squamous cell carcinoma, durvalumab plus tremelimumab combined with proton therapy was shown to have encouraging antitumor efficacy and tolerable safety profiles.[42] Indeed, some clinical trials have already been ongoing or are recruiting patients, and within a few years, the role of PBT in combination with cancer immunotherapy is expected to be defined.