PD-1 Inhibitors Combined With VEGF Inhibitors for Locally Advanced dMMR/MSI-H Colorectal Cancer

Inclusion Criteria: * Locally confirmed dMMR or MSI-H colorectal carcinoma * Tumor staging based on CT/MR or transrectal ultrasound imaging: * Colon cancer: radiological high risk (rT4 or rT3 tumour with extramural extension ≥ 5mm with or without lymph node involvement) * Rectal cancer: \<12 cm from the anal verge and radiological high risk (rT3/4 with or without lymph node involvement) * No sign of bowel obstruction, or bowel obstruction has been relieved by ostomy * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 within 10 days prior to study start * Aged 18 or over * Life expectancy of at least 2 years * Measurable disease * Female participants of childbearing potential must be willing to use adequate contraception for the course of the study starting with the first dose of study medication through 120 days after the last PD-1 antibody dose * Male participants must agree to use adequate contraception for the course of the study starting with the first dose of study medication through 120 days after the last PD-1 antibody dose * Adequate organ function Exclusion Criteria: * Active autoimmune disease that has required systemic treatment in past 2 years * Diagnosis of immunodeficiency or receiving systemic steroid therapy or any other form of immunosuppressive therapy within 2 weeks prior to this study start * Currently participating and receiving treatment in another study within 4 weeks of study start * History of severe allergic reaction to monoclonal antibody * Strong evidence of distant metastases or peritoneal nodules (M1) * Colonic obstruction that has not been defunctioned * Has received prior therapy with an immune checkpoint inhibitor (e.g., anti-programmed cell death \[PD\]-1, anti-PD ligand 1 \[L1\], anti-PD-L2 agent, or anti-cytotoxic T-lymphocyte-associated protein 4 \[CTLA-4\] agent, etc.) or anti-VEGF agents (e.g., Bevacizumab, Apatinib) * Any other malignant disease within the preceding 5 years with the exception of non-melanomatous skin cancer, carcinoma in situ and early stage disease with a recurrence risk \<5% * Received a live vaccine within 30 days of planned start of study medication * Known history of Human Immunodeficiency Virus (HIV), Hepatitis B or C * Known history of, or any evidence of interstitial lung disease or active, non-infectious pneumonitis * Known history of active tuberculosis (Bacillus tuberculosis \[TB\]) * Active infection requiring systemic therapy