Low Rectal Cancer Treated With Total Neoadjuvant Therapy Plus Concurrent Tislelizumab Immunotherapy

Inclusion Criteria: 1. Able to provide written informed consent, understand and comply with the requirements and evaluation schedule 2. ≥18, ≤75 years old 3. Histologically confirmed rectal adenocarcinoma 4. immunohistochemistry confirmed pMMR (positive for MLH1, MSH2, MSH6 and PMS2), or PCR /NGS confirmed MSI-L or MSS 5. The tumor is within 3 cm of the dentate line. through colonoscopy, digital anal examination or MRI 6. clinical stage cT1-3 N 0-1M0 (the 8th UICC/AJCC; T and N is evaluated by MRI) 7. Resectable primary tumor assessed by the Investigator 8. Have not received any anti-tumor treatment for rectal cancer 9. ECOG PS ≤ 1 10. Adequate organ function 11. Female subjects with the ability to become pregnant must have a serum pregnancy test with a negative result within 72 hours before the first dose, and be willing to use highly effective contraceptive methods during the trial and 120 days after the last dose. Male subjects whose partners are women of childbearing potential should be surgically sterilized or agree to use a highly effective method of contraception during the trial and for 120 days after the last dose. Exclusion Criteria: 1. Histologically confirmed poorly differentiated/undifferentiated adenocarcinoma, mucinous adenocarcinoma and signet ring cell carcinoma 2. Have received any treatments for rectal cancer, or evidence of distant metastasis 3. Presence of following high risk factors assessed by MRI: MRF +, EMVI+, cN2, Positive lateral lymph nodes, T3d 4. Presence or in high risk of obstruction, perforation or bleeding; 5. Not suitable for long-course radiotherapy 6. Cannot tolerate surgery 7. ≥2 colorectal cancer lesions at the same time 8. Contraindications for MRI examination 9. Other malignant tumors in the past or at the same time 10. Have an active autoimmune disease requiring systemic therapy within the past 2 years 11. HIV infection 12. Untreated chronic hepatitis B or chronic hepatitis B virus (HBV) carriers (HBV DNA \> 500 IU/mL) or active HCV carriers with detectable HCV RNA; 13. Hypersensitivity to any ingredient of tislelizumab, capecitabine, and oxaliplatin or to any component of the container 14. Other conditions judged by the researcher that do not meet the enrollment requirements