Long-course Chemoradiotherapy or Short-course Radiotherapy Combined With CAPOX, PD-1antibody, and COX-2 Inhibitor for MSS Locally Advanced Rectal Cancer (SERRAC)

Inclusion Criteria: 1. Age 18-75 years, gender not limited 2. Pathologically confirmed rectal adenocarcinoma 3. ≤10 cm from the anus 4. Baseline stage T3-4/N+ 5. No distant metastasis 6. MSI/MMR status MSS/pMMR 7. Karnofsky performance status score ≥70 8. No prior chemotherapy or other anti-cancer treatment prior to enrollment 9. No prior immunotherapy prior to enrollment 10. Ability to comply with the study protocol 11. Written informed consent Exclusion Criteria: 1. Pregnancy or breast-feeding women; 2. Known history of other malignancies within 5 years; 3. Known history of previous anti-tumor treatment, including radiotherapy, chemotherapy, immune checkpoint inhibitors, T cell-related therapy, etc; 4. Known history of severe neurological or mental illness (such as schizophrenia, dementia or epilepsy); 5. Current severe cardiac disease (cardiac dysfunction and arrhythmia), renal dysfunction and liver dysfunction; 6. Acute cardiac infarction or cerebral ischemic stroke occurred within 6 months before recruitment; 7. Uncontrolled infection which needs systemic therapy; 8. Active autoimmune disease or immunodeficiencies, known history of organ transplantation or systematic use of immunosuppressive agents; 9. Known history of human immunodeficiency virus (HIV) infection (i.e., HIV 1 to 2 antibody positive), active syphilis infection, active pulmonary tuberculosis infection 10. Allergic to any component of the therapy.