Inclusion Criteria:
* 1\. Age 18-75 years, of either sex;
* 2\. Stage II/III disease (cT3-T4N0 or cT2-4N+, without distant metastasis, per the 8th edition of the AJCC Cancer Staging Manual, 2018) as assessed by magnetic resonance imaging (MRI) or endoscopic ultrasound (EUS);
* 3\. The lower border of the lesion located ≤10 cm from the anal verge, as assessed by fiberoptic colonoscopy or digital rectal examination;
* 4\. Histologically confirmed or centrally reviewed rectal adenocarcinoma;
* 5\. Eastern Cooperative Oncology Group (ECOG) performance status of 0-1;
* 6\. Hematologic and biochemical parameters meeting the following criteria within 7 days prior to enrollment:
1. Hemoglobin ≥90 g/L; absolute neutrophil count (ANC) ≥1.5×10⁹/L; white blood cell count (WBC) ≥3.5×10⁹/L; platelet count ≥100×10⁹/L (patients must not have received blood transfusion or growth factor support within 14 days prior to blood sample collection);
2. Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤2.5× the upper limit of normal (ULN); alkaline phosphatase (ALP) ≤2.5× ULN;
3. Serum total bilirubin \<1.5× ULN (patients with Gilbert's syndrome may be enrolled if total bilirubin \<3× ULN);
4. Serum creatinine \<1.5× ULN or estimated glomerular filtration rate (eGFR) ≥60 mL/min/1.73 m²;
5. Serum albumin ≥30 g/L;
6. International normalized ratio (INR) or prothrombin time (PT) ≤1.5× ULN, unless the patient is receiving anticoagulant therapy and the PT value is within the expected therapeutic range of the anticoagulant;
7. Activated partial thromboplastin time (APTT) ≤1.5× ULN.
* 7\. Left ventricular ejection fraction (LVEF) ≥ the lower limit of normal (50%), as assessed by Doppler echocardiography;
* 8\. No history of allergy to 5-fluorouracil (5-FU)-based agents or platinum-based agents;
* 9\. Patients with primary rectal cancer must not have received surgery (except palliative ostomy), chemotherapy, or other anti-tumor therapy from the time of diagnosis to enrollment;
* 10\. No prior radiotherapy to the planned radiation field;
* 11\. Written informed consent provided.
Exclusion Criteria:
* 1\. Prior treatment with anti-PD-1/PD-L1 or anti-CTLA-4 immunotherapeutic agents, or other investigational immunotherapy drugs;
* 2\. Patients with severe autoimmune diseases: active inflammatory bowel disease (including Crohn's disease and ulcerative colitis), rheumatoid arthritis, scleroderma, systemic lupus erythematosus, autoimmune vasculitis (e.g., granulomatosis with polyangiitis), etc.;
* 3\. Symptomatic interstitial lung disease or active infectious/non-infectious pneumonitis;
* 4\. Patients with risk factors for bowel perforation: active diverticulitis, intra-abdominal abscess, gastrointestinal (GI) obstruction, abdominal cancer, or other known risk factors for bowel perforation;
* 5\. History of other malignancies, excluding curable non-melanoma skin cancer and carcinoma in situ of the cervix;
* 6\. Patients with active infection, heart failure, myocardial infarction within the past 6 months, unstable angina, or unstable arrhythmia;
* 7\. Physical examination or clinical laboratory findings, or other uncontrolled diseases that, in the investigator's judgment, may interfere with the study results or increase the risk of treatment complications for the patient;
* 8\. Women who are pregnant or breastfeeding;
* 9\. Congenital or acquired immunodeficiency disorders, including human immunodeficiency virus (HIV) infection, or a history of organ transplantation or allogeneic stem cell transplantation;
* 10\. Known active hepatitis B virus (HBV) infection (HBV-DNA ≥2000 U/mL), hepatitis C virus (HCV) infection, or active pulmonary tuberculosis;
* 11\. Patients who have previously received a tumor vaccine, or who have received other vaccines within 4 weeks prior to initiation of study treatment (Note: injectable seasonal influenza vaccines, which are mostly inactivated vaccines, are permitted; intranasal formulations, which are usually live attenuated vaccines, are not permitted);
* 12\. Patients concurrently receiving other immunomodulatory agents, chemotherapeutic agents, investigational drugs from other clinical studies, or requiring long-term corticosteroid therapy are not eligible for enrollment;
* 13\. Patients with psychiatric disorders, substance abuse, or social issues that may affect compliance are excluded at the discretion of the investigator;
* 14\. Patients with allergy to or contraindications for the study treatment drugs.
