Node-sparing Radiotherapy Combined with Total Neoadjuvant CAPOX and Sintilimab for MSS Middle and Low Rectal Cancer

Inclusion Criteria: * Patients who have a strong willingness to preserve the anus and are willing to receive neoadjuvant therapy. * Male or Female aged 18-75. * Patients diagnosed with low rectal cancer within 10 cm from the lower edge of the tumor to the anal verge by pelvic MRI and anorectoscopy, the clinical stage is cT2N+M0/cT3-4aN0/+M0, the lymph nodes are limited to the mesorectum, the circumferential resection margin is negative. * Histologically confirmed rectal adenocarcinoma; Genetic testing suggests MSI-L or MSS, or tumor biopsy immunohistochemistry reveals pMMR, that is, MSH1, MSH2, MSH6, and PMS2 are all positive. * Eastern Cooperative Oncology Group (ECOG) 0-1. * No previous treatment(including anti-tumor therapy、immunotherapy or pelvic radiation). * Adequate hematologic, hepatic, renal, thyroid and cardiac function: white blood cells ≥3500/mm3, neutrophils ≥1800/mm3, platelets ≥100,000/mm3, hemoglobin ≥100 g/L; activated partial thromboplastin time, prothrombin time and international normalized ratio ≤1.5 × ULN; aspartate aminotransferase and alanine aminotransferase ≤3.0 × upper limit of normal (ULN), bilirubin ≤1.25 × ULN, serum albumin ≥28 g/L. creatinine clearance ≥50 mL/mi, creatinine ≤1.5 × ULN; * Informed consent form signed. Exclusion Criteria: * Patients with a previous history of malignant tumors besides rectal cancer. * Patients with distant metastases before enrollment. * Patients with positive internal or external iliac lymph nodes are assessed by MRI or CT. * Patients with obstruction, perforation, or bleeding that require emergency surgery. * Patients with severe concomitant diseases and estimated survival time ≤ 5 years. * Allergic to any component of the therapy. * Patients with poorly differentiated adenocarcinoma, signet ring cell carcinoma, or mucinous adenocarcinoma. * Patients who received immunosuppressive or systemic hormone therapy for immunosuppressive purposes within 1 month prior to the initiation of therapy. * Patients who have received any other experimental drug (including immunotherapy) or participated in another interventional clinical trial within 30 days before screening. * Factors leading to study termination, such as alcoholism, drug abuse, other serious illnesses (including psychiatric disorders) requiring combination therapy, and patients with severe laboratory abnormalities. * Patients with congenital or acquired immune deficiency (such as HIV infection). * Vulnerable groups, including mentally ill, cognitively impaired, critically ill patients, minors, pregnant or lactating women, illiterate, etc. Other conditions that investigators consider not suitable for this study.