This phase II/III trial studies how well neoadjuvant short-course radiotherapy and chemotherapy with or without PD-1 inhibitors works in treating patients with locally advanced rectal adenocarcinoma. Neoadjuvant short-course radiation therapy followed by two-drug regimen chemotherapy, such as CAPOX, were shown to be non-inferior to standard long-course chemoradiotherapy in our previous STELLAR study. Immune checkpoint inhibitors (ICIs) using monoclonal antibodies, such as PD-1 or PD-L1 inhibitor, show promising efficiency and reliable security in some limited sample prospective or retrospective studies. When treating patients with locally advanced rectal cancer, giving sequential neoadjuvant short-course radiotherapy and chemotherapy with PD-1 inhibitor may work better.
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;
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.