Short-course Radiotherapy Followed by Chemotherapy and PD-1 Inhibitor for Locally Advanced Rectal Cancer

Inclusion Criteria: * Biopsy proven rectal adenocarcinoma; * Distance between tumour and anal verge≤ 10cm; * Locally advanced tumour;(8th edition AJCC/UICC staging :cT3-T4N0/cT2-4N+，M0) Cancer Staging must be based on pelvic MRI or Endoscopic ultrasound; * Eastern Cooperative Oncology Group(ECOG) performance score ≤ 1; * Mentally and physically fit for chemotherapy; Adequate blood counts: White blood cell count ≥3.5 x 109/L Haemoglobin levels ≥100g/L Platelet count ≥100 x 109/L Creatinine levels ≤1.0× upper normal limit（UNL） Urea nitrogen levels ≤1.0× upper normal limit（UNL） Alanine aminotransferase(ALT) ≤1.5× upper normal limit（UNL） Aspartate aminotransferase(AST) ≤1.5× upper normal limit（UNL） Alkaline phosphatase(ALP) ≤1.5× upper normal limit（UNL） Total bilirubin(TBIL) ≤1.5× upper normal limit（UNL） * No excision of tumor, chemotherapy or other anti-tumor treatment after the diagnosis. * No previous pelvic radiation history； * Written informed consent; Exclusion Criteria: * Previous treatment with anti-PD-1/L1 and anti-CTLA-4 or other immune experimental drugs. * Severe autoimmune disease: active inflammatory bowel disease (including Crohn's disease, ulcerative colitis), rheumatoid arthritis, scleroderma, systemic lupus erythematosus, autoimmune vasculitis (e.g. Wegener's granulomatosis) * Symptomatic interstitial lung disease or active infectious/non-infectious pneumonia. * At risk for bowel perforation: active diverticulitis, intra-abdominal abscess, gastrointestinal (GI) obstruction, abdominal cancer or other known risk factors for bowel perforation. * history of other malignancies, excluding curable non-melanotic skin cancer and cervix carcinoma in situ； * Active infection, heart failure, heart attack within 6 months, unstable angina or unstable arrhythmia. * Any condition investigator considered may interfere with the results or place the patient at increased risk of treatment complications, or other uncontrollable disease. * Pregnancy or breast feeding * Immunodeficiency disorders including human immunodeficiency virus (HIV), or history of organ transplantation, allogeneic stem cell transplantation * Active hepatitis B virus (HBV) hepatitis (HBV-DNA ≥ 2000 U/mL), hepatitis C virus (HCV) hepatitis, active tuberculosis infection. * Oncology vaccination history or any vaccination within 4 weeks prior to the start of treatment.(Note: influenza vaccines are mostly inactivated and therefore allowed, intranasal preparations are usually live attenuated vaccines and therefore not allowed) * Concomitant other immune agents, chemotherapeutic agents, other drugs in clinical studies, and long term cortisol application