SBRT Combined With CAPEOX, Bevacizumab, and PD-1 Inhibitor for the Treatment of RAS-Mutant, MSS-Type, Unresectable Metastatic Colorectal Cancer.

Inclusion Criteria: * Aged 18-75 years, regardless of gender. * Lower edge of the lesion located ≥12 cm from the anal verge. * Histologically confirmed colorectal adenocarcinoma. * Confirmed as unresectable by multidisciplinary team (MDT) evaluation. * RAS mutation-positive. * Microsatellite/mismatch repair status: MSS/pMMR. * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1. * Expected survival ≥3 months. * Adequate hematological, hepatic, and renal function: Neutrophil count ≥1.5 × 10⁹/L; Platelet count ≥75 × 10⁹/L; Serum total bilirubin ≤1.5 × upper normal limit (UNL); Aspartate aminotransferase (AST) ≤2.5 × UNL; Alanine aminotransferase (ALT) ≤2.5 × UNL; Serum creatinine ≤1.5 × UNL. * Karnofsky Performance Status (KPS) score ≥70. * Adequate organ function with no contraindications to surgery, radiotherapy, chemotherapy, or immunotherapy. * No prior chemotherapy or other antitumor therapy before enrollment. * No prior immunotherapy received. * Willingness and ability to comply with the study protocol during the trial period. * Signed written informed consent. Exclusion Criteria: * Patients who have received antibodies against programmed death receptor-1 (PD-1) or its ligand (PD-L1), as well as antibodies against cytotoxic T lymphocyte associated antigen 4 (CTLA-4). * Patients with any active autoimmune diseases or a history of requiring steroid or immunotherapy treatment. * Complex situations with concurrent active bleeding, perforation, or requi…