Node-Sparing Short-Course Radiotherapy Plus Chemotherapy, Bevacizumab and PD-1 Inhibitor in Metastatic pMMR/MSS Colorectal Cancer (MODIFI-CRC)

Inclusion Criteria: * Voluntarily signs a written informed consent form. * Aged between 18 and 75 years at the time of enrollment. * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1. * Expected survival of more than 3 months. * Histologically or cytologically confirmed colorectal adenocarcinoma. * Patients must be considered unsuitable for curative surgical resection or local treatment and must not have received prior systemic anti-tumor therapy for recurrent or metastatic disease. Patients with prior neoadjuvant or adjuvant therapy may be enrolled if recurrence or metastasis occurs ≥12 months after the last dose of such treatment. * At least one measurable lesion per RECIST v1.1 that can be accurately measured repeatedly. Note: Brain metastases cannot be used as target lesions. * Able to provide 10-20 unstained tumor tissue FFPE slides from recent biopsy or archival material stored within 3 years. Ten slides will be used for immunohistochemistry and ten for genomic testing (recent biopsy preferred). If samples are of insufficient quality (as determined by the central lab), additional slides may be required. If adequate slides cannot be provided, partial or full waiver may be granted upon approval by the medical monitor. * Agrees to provide tumor tissue and peripheral blood samples during screening and throughout the study for research purposes. * Adequate organ function as defined below: * Hematologic (no use of blood products or growth factors within 7 days prior to treatment): * Absolute neutrophil count (ANC) ≥ 1.5 × 10⁹/L * Platelet count ≥ 100 × 10⁹/L * Hemoglobin ≥ 90 g/L * Renal: * Calculated creatinine clearance (CrCl) ≥ 50 mL/min (Cockcroft-Gault formula: CrCl = \[(140 - age) × weight (kg) × 0.85 (if female)\] / \[72 × serum creatinine (mg/dL)\]) * Urine protein \< 2+ on dipstick or \< 1.0 g per 24-hour urine collection * Hepatic: * Total bilirubin (TBil) ≤ 1.5 × upper limit of normal (ULN) * AST and ALT ≤ 2.5 × ULN * Serum albumin ≥ 28 g/L * Coagulation: * International normalized ratio (INR) and activated partial thromboplastin time (APTT) ≤ 1.5 × ULN * Cardiac: * Left ventricular ejection fraction (LVEF) ≥ 50% * Women of childbearing potential must have a negative urine or serum pregnancy test within 3 days prior to initiating study treatment. If the urine test is inconclusive, a serum test must confirm the result. Women of childbearing potential engaging in sexual activity with non-sterilized male partners must agree to use acceptable contraception from screening through 120 days after the last dose of study drug. The decision to discontinue contraception beyond this period should be discussed with the investigator. Periodic abstinence and the rhythm method are not acceptable. * Women of childbearing potential are defined as those who have not undergone surgical sterilization (bilateral tubal ligation, bilateral oophorectomy, or hysterectomy) and have not been postmenopausal (defined as at least 12 consecutive months of amenorrhea without an alternative medical cause and FSH levels in the postmenopausal range). * Highly effective contraception methods are those with a failure rate of \<1% per year when used consistently and correctly. In addition to barrier methods, hormonal contraception (e.g., oral contraceptives) must also be used. * Willing and able to comply with scheduled visits, treatment procedures, laboratory tests, and other protocol requirements. Exclusion Criteria: * • Known MSI-H or dMMR status. * History of other malignancies within the past 3 years, except for those cured by local treatment, such as basal cell carcinoma, squamous cell carcinoma of the skin, superficial bladder cancer, or ductal carcinoma in situ of the breast. * Prior immunotherapy, including immune checkpoint inhibitors (e.g., anti-PD-1, anti-PD-L1, anti-CTLA-4 antibodies), immune agonists (e.g., ICOS, CD40, CD137, GITR, OX40), or immune cell-based therapies targeting tumor immunity. * Prior adjuvant or neoadjuvant therapy targeting EGFR or VEGF/VEGFR pathways (e.g., bevacizumab, cetuximab, panitumumab, aflibercept, regorafenib, or biosimilars). * Active CNS metastases. Patients with previously treated brain metastases may be eligible if clinically stable for at least 2 weeks (from the first dose of study drug) and off corticosteroids for at least 3 days prior to treatment. Patients with untreated, asymptomatic brain metastases (no neurologic symptoms, no corticosteroids, and all lesions ≤ 1.5 cm in longest diameter) may enroll but require regular CNS monitoring. * Known brainstem, meningeal, or spinal cord metastases or compression. * Symptomatic or recurrently drained pleural effusion, pericardial effusion, or ascites. * Prior systemic or local anti-tumor therapy for locally advanced rectal cancer, including curative surgery, chemotherapy, radiotherapy, immunotherapy, biologics, or small-molecule targeted therapy. * Receipt of nonspecific immunomodulators (e.g., interleukins, interferons, thymic peptides, TNF) within 2 weeks prior to study treatment (except IL-11 for thrombocytopenia), or anti-tumor traditional Chinese medicine within 1 week prior to study treatment. * Active autoimmune disease requiring systemic treatment within the past 2 years (e.g., with corticosteroids, immunosuppressants, or DMARDs). Replacement therapy (e.g., thyroid hormone, insulin, or physiologic corticosteroids for adrenal/pituitary insufficiency) is allowed. * History of non-infectious pneumonitis requiring systemic corticosteroids, or current interstitial lung disease. * History of bleeding disorders or coagulopathy; long-term anticoagulation (e.g., atrial fibrillation with CHADS2 score ≥ 2). * Uncontrolled comorbidities including, but not limited to, decompensated cirrhosis, nephrotic syndrome, uncontrolled metabolic disorders, severe active peptic ulcer disease or gastritis, or psychiatric/social condit