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/cT3-4N0/+M0, the lymph nodes are limited to the mesorectum. * 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, perforat…