Adjuvant PD-1 Blockade for High-risk Stage-II DMMR/MSI-H Colorectal Cancer

This is an open-label, multi-centre, randomised, phase III trial comparing the combination of PD-1 blockade + SOC versus SOC alone as adjuvant therapy for patients with high-risk stage-II dMMR/MSI-H colorectal cancer. Primary Objective: To determine whether the addition of Tislelizumab can significantly improve disease-free survival (DFS) compared to standard of care in patients with high-risk stage-II colorectal cancer. Secondary objectives: * To determine whether the addition of Tislelizumab can significantly improve overall survival (OS) compared to standard of care * To assess the adverse events (AE) profile (including immune-related adverse events, ir-AEs). OUTLINE: Patients are randomized to 1 of 2 arms, stratified by cT4 status. Arm 1 (experimental group): patients receive Tislelizumab 200mg intravenously on day 1, with or without adjuvant chemotherapy, and repeat the treatment on day 22. Patients undergo routine follow-up every 3 months for the first 3 years, and then every 6 months for the year 4-5. Arm 2 (control group): patients receive standard of care (SOC), whether with surveillance alone, single-agent Capecitabine, or CapeOx/FOLFOX, at the discretion of the doctor in charge. Patients undergo routine follow-up every 3 months for the first 3 years, and then every 6 months for the year 4-5. Statistics According to the statistical design, 180 patients (90 per arm) are to be randomized. The study is expected to take up to 36 months to complete accrual.