The establishment of an effective strategy for recurrence prevention following curative treatment for hepatocellular carcinoma (HCC), including radiotherapy, remains a critical unmet clinical need. Despite favourable local control and safety profiles, recurrence after particle therapy remains a major challenge, highlighting the need for effective adjuvant strategies to improve long-term outcomes. The present phase Ib/II trial is designed to evaluate the safety and efficacy of atezolizumab plus bevacizumab (Atezo+Bev) administered after carbon-ion radiotherapy (C-ion RT) in patients with unresectable non-metastatic large HCC. This study aims to explore the potential of this multimodal approach as a novel adjuvant strategy to improve outcomes in patients at high risk of recurrence.

This is a phase Ib/II, single-arm clinical trial designed to evaluate the safety and efficacy of adjuvant Atezo+Bev following C-ion RT in patients with HCC. Eligible patients will be enrolled in the first registration phase. C-ion RT (60 Gy) will be administered over four consecutive treatment days ideally within one calendar week. Patients will receive a combination of atezolizumab (1200 mg) and bevacizumab (15 mg/kg) administered intravenously every 21 days for one treatment cycle. The primary endpoint of the phase Ib part is the proportion of patients with dose-limiting toxicity (DLT). DLT is defined as prespecified toxicities associated with the investigational drug among the adverse events that occurred from the start date of the investigational drug (Day 1) to Day 21. If there is one or fewer cases of DLT out of six cases, the trial will proceed to the phase II part. The primary endpoint of the phase II part is the 1-year recurrence-free survival rate.

This study was approved by the ethics committee of two participating institutions (Chiba University Hospital (approval No. 2024021) and National Institute for Quantum and Radiological Science and Technology, QST Hospital (approval No. C24-001)). Trial results will be reported in a peer-reviewed journal publication.

jRCT2031240284.