SCRT-NALIRIXELOX+Sintilimab as TNT for High-Risk LARC

Inclusion Criteria: * Voluntary Participation: Subjects voluntarily participate in the study, sign the informed consent form, and have good compliance. * Age and Gender: Age between 18 and 75 years, any gender. * Pathological Diagnosis: Histologically or cytologically confirmed rectal adenocarcinoma; confirmed mismatch repair proficient (pMMR) or microsatellite stable (MSS) by genetic testing or immunohistochemistry. * Disease Stage: Assessed as high-risk, mid-low (distance from the anal verge ≤10 cm) locally advanced rectal cancer by colonoscopy, digital rectal exam, or MRI. High-risk definition: Meeting at least one of the following: cT4, extramural vascular invasion (EMVI), tumor deposits, involvement of the mesorectal fascia or intersphincteric plane. * Prior Treatment: No prior anti-tumor therapy for this rectal cancer (including surgery, radiotherapy, chemotherapy, immunotherapy, or targeted therapy). * Measurable Lesion: At least one measurable lesion according to RECIST v1.1 criteria. * Tumor Tissue Sample: Must provide tumor tissue samples for biomarker analysis. * Adequate Organ Function (within 7 days before the first dose): 1) Hematological: Hemoglobin ≥90 g/L; White Blood Cell count ≥3.0 x 10⁹/L; Absolute Neutrophil Count ≥1.5 x 10⁹/L; Platelets ≥100 x 10⁹/L. 2) Hepatic: Total Bilirubin \<1.5 x ULN; AST and ALT ≤2.5 x ULN; Albumin ≥30 g/L. 3) Renal: Serum Creatinine ≤1.5 x ULN OR Creatinine Clearance ≥50 mL/min. 4) Coagulation: INR ≤1.5 x ULN (or ≤3 x ULN if on …