This study is a prospective, observational, real-world investigation. This study will evaluate the efficacy and safety of tislelizumab monotherapy before surgery in patients with mismatch repair deficient or microsatellite instability high (dMMR/MSI-H) locally advanced colorectal cancer.

All patients will receive three cycles of tislelizumab neoadjuvant therapy followed by curative surgery. Postoperatively, based on surgical pathology, patients will receive adjuvant therapy with a regimen selected by the investigator or adopt a watch-and-wait strategy. The investigators will conduct a 5-year prospective follow-up.

The investigators plan to enroll approximately 30 subjects.

Inclusion Criteria:

1. Age ≥ 18 years, ECOG performance status score 0-2.
2. Pathologically confirmed stage II-III colorectal adenocarcinoma (according to AJCC 8th edition).
3. Histologically confirmed mismatch repair deficient or genetic testing confirmed microsatellite instability high tumor.
4. Signed written informed consent.

Exclusion Criteria:

1. The patient has a history of other serious conditions rendering them unsuitable for surgery.
2. Subjects with the following conditions: active autoimmune diseases, active infectious diseases, inflammatory bowel diseases; requiring long-term glucocorticoid or immunosuppressive therapy during treatment; history of immunodeficiency; history of organ transplantation or haematopoietic stem cell transplantation; severe interstitial pneumonia or pulmonary fibrosis.
3. Known hypersensitivity to any component or excipient of tislelizumab or other PD-1/PD-L1 agents.
4. Other conditions deemed unsuitable for inclusion by the investigator.

Patients with colorectal adenocarcinoma registered and treated at the Cancer Hospital Chinese Academy of Medical Sciences.