This study is designed as a single-center, phase II exploratory clinical trial to evaluate the pathological response rate and safety of sintilimab (PD-1 inhibitor) combined with XELOX regimen (oxaliplatin + capecitabine) for neoadjuvant treatment of LARC. The study focuses on the pMMR/MSS patient population and attempts to provide a new treatment option for patients who cannot tolerate radiotherapy or need to preserve organ function.

Inclusion Criteria:1. Pathologically confirmed rectal adenocarcinoma (non-special type and special type, including mucinous adenocarcinoma, signet ring cell carcinoma, serrated adenocarcinoma, micropapillary carcinoma, medullary carcinoma, cribriform comedo adenocarcinoma); preoperative imaging confirmed that the stage is 1) rectal cancer T3 and (or) N+ resectable rectal cancer patients; 2) for T4 patients, re-evaluation of resectability must be conducted after the completion of neoadjuvant therapy, and radiotherapy may be used in combination depending on the situation. 2. Age ≥ 18 years old, gender is not limited, ECOG score 0-2 points; 3. Physical condition and organ function allow for major abdominal surgery/4. Agree that the researchers use blood, stool and pathological sections for research during the study; 5. Major organ function meets the following requirements (laboratory test values within 7 days before enrollment must meet the following standards) ① Routine blood examination: (no blood transfusion, no use of granulocyte colony stimulating factor, no use of drug correction within 14 days before screening): a) Neutrophil ≥ 1.5×109/L; b) Platelet ≥ 75×109/L; c) Hemoglobin ≥ 90g/L; ② Biochemical examination: (no albumin transfusion within 14 days before screening): a) Serum creatinine ≤ 1.5×upper limit of normal (ULN), or creatinine clearance> 50 mL/min; b) Serum total bilirubin ≤ 1.5×ULN; c) Aspartate aminotransferase (AST), alanine aminotransferase (ALT) ≤ 2.5 × ULN; ③ Coagulation function: a) International normalized ratio (INR) ≤ 2.3 or prothrombin time (PT) exceeds the normal control range ≤ 6 seconds. 6. Patients are recruited regardless of their programmed death ligand 1 (PD-L1) expression level. -

Exclusion Criteria:1) The tumor has distant metastasis or peritoneal metastasis confirmed by laparoscopy; 2) Patients with other severe immunosuppressive diseases or other malignant tumors; 3) Any condition that affects the intestinal absorption of capecitabine; 4) Severe uncontrolled recurrent infections, or other serious uncontrolled concomitant diseases; severe mental illness; severe respiratory diseases; severe liver and kidney dysfunction; history of unstable angina or myocardial infarction within 6 months; history of cerebral infarction or cerebral hemorrhage within 6 months; continuous use of glucocorticoids within 1 month (except local application); 5) The researcher determines that the patient has other complications that endanger the patient's safety or may affect the completion of the study; 6) Currently or previously received other research drugs or any immunotherapy; 7) Suffering from malignant tumors in other parts within five years; 8) The patient is pregnant or lactating;

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