Total Neoadjuvant Treatment for pMMR Locally Advanced Low Rectal Cancer: Anti-PD-1 Antibody With FOLFOXIRI and Preoperative Radiation Therapy

Inclusion Criteria: The pathological confirmation is rectal adenocarcinoma. Additionally, the tumor biopsy immunohistochemistry indicates pMMR, with all MSH1, MSH2, MSH6, and PMS2 showing positive expression. Age: 18 to 75 years old,allgenders MRI examination confirms that the lower margin of the tumor is within 5cm from the anal verge. Clinical stage cT3-4N0M0 or cTxN+M0, with or without positive MRF, with or without positive EMVI, estimated to be eligible for R0 resection. No signs of intestinal obstruction before surgery; or resolution of obstruction after proximal colostomy surgery Performance status (ECOG score) ≤ 2 Hematology: WBC > 3 × 10^9 / L; PLT > 80 × 10^9 / L; Hb > 90 g/L; Liver function: ALT and AST ≤ 2.5 × ULN; bilirubin ≤ 1.5 × ULN; Renal function: Serum creatinine ≤ 1.5 × ULN or creatinine clearance rate (CCr) ≥ 60 ml/min; Signed informed consent, willingness to undergo treatment according to this protocol, and good compliance with medication. No history of prior surgery, chemotherapy, radiation therapy, or immunotherapy. Exclusion Criteria: Co-existing with other serious illnesses, including severe electrolyte disorders, bleeding tendencies, etc. Active or uncontrolled severe infections: a) Known human immunodeficiency virus (HIV) infection. b) Known clinically significant liver disease history, including viral hepatitis [known carriers of hepatitis B virus (HBV) must exclude active HBV infection, i.e., HBV DNA positive (>1×104 copies/mL or >2000 IU/mL)]. c) Known hepatitis C virus (HCV) infection with positive HCV RNA (>1×103 copies/mL), or other hepatitis, liver cirrhosis. Women who are pregnant or breastfeeding and have childbearing potential but are not taking adequate contraceptive measures. Patients with severe brain disorders or mental illnesses (such as depression, mania, obsessive-compulsive disorder, and schizophrenia) that affect the patient's ability to self-report. Patients with autoimmune diseases, blood system disorders, and a history of organ transplantation, long-term use of steroids, or immunosuppressive agents History of other malignant tumors within the past 5 years, excluding cured cervical carcinoma in situ or basal cell carcinoma of the skin. History of organ transplantation (including autologous bone marrow transplantation and peripheral stem cell transplantation). Hypertension that cannot be well controlled with antihypertensive medications (systolic blood pressure ≥150 mmHg or diastolic blood pressure ≥100 mmHg).