Different Cycles of Preoperative Neoadjuvant Sintilimab in Mismatch-repair Deficient/microsatellite Instability-high, Locally Advanced Colorectal Cancer

Inclusion Criteria: * has pathologically confirmed colon cancer or upper rectal cancer (with the lower edge of the tumor more than 10 cm from the anal verge); clinical stage cT3/T4 or N+ * no urgent need for surgery due to bleeding, perforation, or obstruction * immunohistochemistry of tumor biopsy indicating deficient mismatch repair (dMMR), defined by the loss of expression of one or more of the four proteins MSH1, MSH2, MSH6, and PMS2; or genetic testing indicating MSI-H Exclusion Criteria: * Has distant metastatic disease. * Has received prior medical therapy (chemotherapy, immunotherapy, biologic, or targeted therapy), radiation therapy or surgery for management of colon cancer * Has undergone any major surgical procedure, open biopsy, or experienced significant traumatic injury within 28 days prior to randomization * Is receiving any other anticancer or experimental therapy. No other experimental therapies (including but not limited to chemotherapy, radiation, hormonal treatment, antibody therapy, immunotherapy, gene therapy, vaccine therapy, or other experimental drugs) of any kind are permitted while the participant is receiving study intervention * There is a history of illness or evidence of disease, treatment, or abnormal laboratory test values that could potentially interfere with the trial results or hinder the subject's full participation in the study, or any other conditions deemed unsuitable for inclusion by the investigator. * The investigator believes there are other potential risks that make the subject unsuitable for participation in this study.