Due to dMMR colon cancer patients respond poorly to conventional chemotherapy, but immunotherapy can significantly improve the pCR in this group of patients, this study intends to explore whether neoadjuvant immunotherapy can improve the R0 resection rate with preservation of adjacent organs in T4 colon cancer patients with dMMR.

Inclusion Criteria:

1. Aged 18-75 years;
2. ECOG score 0-2;
3. Adenocarcinoma confirmed by pathology and dMMR confirmed by IHC or PCR;
4. Tumor located in cecum, ascending colon, transverse colon and sigmoid colon;
5. Patients with clinical stage cT4: (1) Loss of space between tumor and adjacent organs or invasion of adjacent organs as assessed by enhanced CT; (2) R0 resection cannot achieve according to intraoperative exploration;
6. No evidence of distant metastasis;
7. Newly treated patients who have not received treatment including chemotherapy and surgery;
8. Liver, kidney and other organs have good function and can tolerate chemotherapy and surgery;
9. Patients and family members can understand the study protocol, voluntarily participate in the study and sign informed consent.

Exclusion Criteria:

1. A history of other malignant tumors (other than cured basal cell carcinoma, cervical carcinoma in situ, surgically treated localized prostate cancer, or surgically resected breast ductal carcinoma in situ) within the past 5 years;
2. Complicated with intestinal obstruction, intestinal perforation, gastrointestinal bleeding and other patients requiring emergency surgery; pregnant or lactating women;
3. Patients with a history of severe mental illness, immune disease, hormone medication;
4. Patients contraindicated by immunotherapy or surgery;
5. Participated in other clinical researchers in the past 3 months;
6. Any other circumstances that the investigator considers inappropriate for inclusion.