Short-Course Radiotherapy Combined With Chemotherapy and Immunotherapy in Mid-Low Locally Advanced Rectal Cancer

Inclusion Criteria: 1. Age 18-75 years. 2. ECOG performance status score 0-2. 3. Rectal adenocarcinoma confirmed by colonoscopic pathology, with pMMR or MSS. 4. Imaging studies confirm no distant metastasis or lateral lymph node metastasis; MRI staging is II/III (excluding T4b, N1c, N2) and any positive lymph nodes (if present) are confined within the mesorectum. 5. MRI shows the distal margin of the tumor is ≤10 cm from the anal verge, and the mesorectal fascia (MRF) is negative. 6. The longest diameter of the rectal cancer lesion is ≥10 mm on baseline CT or MRI (meeting the definition of a "measurable lesion" per RECIST 1.1 criteria). 7. Willing and able to comply with the study procedures. 8. Consent to the use of tissue and blood samples by the investigator for medical research purposes. 9. No prior history of radiotherapy or immunotherapy. 10. No history of immune system diseases (e.g., systemic lupus erythematosus, rheumatoid arthritis, systemic vasculitis, scleroderma, pemphigus, dermatomyositis, mixed connective tissue disease, autoimmune hemolytic anemia, ulcerative colitis, HIV infection, etc.). 11. No history of endocrine system diseases (e.g., hyperthyroidism, hypothyroidism, thyroid nodules, thyroiditis, type 1 diabetes, type 2 diabetes, gestational diabetes, other specific types of diabetes, Cushing's syndrome, primary aldosteronism, pheochromocytoma, adrenal insufficiency, pituitary adenoma, anterior pituitary hypofunction, acromegaly, gigantism, polycystic ovary syndrome, precocious puberty, hypogonadism, hyperparathyroidism, hypoparathyroidism, etc.). 12. No severe cardiac, pulmonary, hepatic, or renal dysfunction. 13. No jaundice or gastrointestinal obstruction. 14. No concurrent acute infection. 15. Subjects must undergo all required baseline laboratory assessments, and results must be obtained within 1 week before enrollment. Laboratory values must meet the following criteria (per CTCAE 5.0): 1. White blood cell count ≥2000/μL. 2. Neutrophil count ≥1500/μL. 3. Platelet count ≥100×10³/μL. 4. Hemoglobin ≥9.0 g/dL. 5. Creatinine: serum creatinine ≤1.5 × upper limit of normal (ULN) or creatinine clearance \>50 mL/min (calculated using the Cockcroft-Gault formula). 6. Creatinine clearance for females = \[140 - age (years)\] × weight (kg) × 0.85 ÷ \[72 × serum creatinine (mg/dL)\]. 7. Creatinine clearance for males = \[140 - age (years)\] × weight (kg) × 1.00 ÷ \[72 × serum creatinine (mg/dL)\]. 8. Aspartate aminotransferase (AST) ≤3 × ULN, alanine aminotransferase (ALT) ≤3 × ULN, total bilirubin ≤1.5 × ULN. 16. No psychiatric/psychological disorders affecting social functioning. 17. Women of childbearing potential must have a negative serum pregnancy test (blood HCG) within 1 week before enrollment. 18. Women of childbearing potential and men who are sexually active with women of childbearing potential must agree to use appropriate contraceptive methods. Exclusion Criteria: 1. Multifocal cancer, or concomitant other malignant tumors. 2. Received any anti-tumor therapy for other malignant tumors within the past 5 years. 3. Underwent major surgery recently (within 6 months). 4. Presence of multiple factors affecting oral drug absorption (e.g., inability to swallow, nausea, vomiting, chronic diarrhea, intestinal obstruction, etc.). 5. Any uncontrolled, severe comorbid diseases. 6. Allergy to any component of the study medication. 7. Life expectancy less than 5 years for any reason. 8. Planning to undergo or previously underwent organ/bone marrow transplantation. 9. Received immunosuppressive agents or glucocorticoid therapy aimed at suppressing immune responses within 1 month before enrollment. 10. For patients with a history of central nervous system diseases, the investigator will determine whether their current condition allows them to provide informed consent or comply with the study procedures, and subsequently decide whether they can be enrolled. 11. Other diseases or conditions that may prevent completion of the study treatment (e.g., alcoholism, drug addiction, etc.). 12. Pregnant or lactating women.