ctDNA-Guided Immunotherapy for dMMR/MSI-H Colon Cancer

Inclusion Criteria:

* Written informed consent obtained before any study-related procedures.
* Age 18 to 75 years, inclusive.
* Histologically confirmed colon adenocarcinoma, mucinous adenocarcinoma, or signet-ring cell carcinoma, with the tumor located at least 15 cm from the anal verge.
* dMMR/MSI-H status confirmed by immunohistochemistry, polymerase chain reaction, or next-generation sequencing.
* Eastern Cooperative Oncology Group performance status of 0 or 1 and an expected survival of at least 3 months.
* Adequate hematologic, hepatic, renal, coagulation, thyroid, and cardiac function, as defined in the protocol.
* Negative pregnancy test for women of childbearing potential; agreement to use highly effective contraception, when applicable.

Exclusion Criteria:

* Prior treatment with a PD-1 inhibitor, CTLA-4 inhibitor, or other immunotherapy.
* Symptomatic or high-risk bowel obstruction, bleeding, perforation, pneumonitis, or other conditions that may compromise safe study participation.
* Another malignancy diagnosed within 5 years before the first study treatment, except for specified definitively treated low-risk malignancies.
* Current participation in another interventional clinical study, or receipt of another investigational drug or investigational device within 4 weeks before the first study treatment.
* Active autoimmune disease requiring systemic treatment within 2 years before the first study treatment, or recent systemic corticosteroid or other immunosuppressive therapy.
* Uncontrolled pleural effusion or ascites, prior allogeneic organ transplantation (except corneal transplantation), or allogeneic hematopoietic stem cell transplantation.
* Known hypersensitivity to any study drug component.
* Toxicities or complications from prior treatment not recovered to Grade 1 or baseline, except for specified conditions.
* HIV infection, untreated active hepatitis B infection, or active hepatitis C infection.
* Receipt of a live vaccine within 30 days before the first study treatment.
* Pregnancy or breastfeeding.
* Any serious or uncontrolled systemic disease, active infection, clinically significant laboratory abnormality, or other condition that may interfere with study participation or place the participant at unacceptable risk, as judged by the investigator.