PD-L1 Inhibitor + RT ± Ursodeoxycholic Acid in Recurrent/Metastatic HER2-Neg Breast Cancer

Inclusion Criteria: 1. Age ≥ 18 years； 2. Patients with recurrent/metastatic HER2-negative breast cancer； 3. Patients who have previously received standard treatment regimens for recurrent/metastatic breast cancer； 4. At least one lesion suitable for radiation therapy； 5. At least one measurable metastatic lesion outside of the radiation field, and can be monitored using the "Response Evaluation Criteria in Solid Tumors" (RECIST) version 1.1； 6. ECOG performance status of 0-2； 7. Signed informed consent； 8. Patients who have previously received radiation therapy may be included as long as it does not interfere with irradiation of the target lesion； Exclusion Criteria: 1. Biliary obstruction, acute or chronic cholecystitis or cholangitis, or long-term biliary colic (contraindication for UDCA)； 2. Malabsorption syndrome or diseases that significantly affect gastrointestinal function; patients who have undergone total gastrectomy or resection of the proximal small intestine that may affect oral drug absorption； 3. Exclusion of patients with symptomatic brain metastases or leptomeningeal metastasis; patients with brain metastases who have been treated and stabilized (with no progression within 4 weeks) may be included, but brain metastases cannot be used as target lesions； 4. Known invasive malignancies within the past 5 years that are still progressing or require active treatment (excluding patients with basal cell carcinoma, squamous cell carcinoma of the skin, or breast ductal carcinoma in situ or cervical carcinoma in situ who have received curative treatment)； 5. Previous immune therapy resulting in grade 3 or higher adverse events； Diagnosed with immunodeficiency or receiving long-term systemic corticosteroid treatment (prednisone equivalent dose \>10 mg daily) or any form of immunosuppressive therapy within 7 days before the first dose of study treatment； 6. Active autoimmune diseases requiring systemic treatment (e.g., using disease-modifying drugs, corticosteroids, or immunosuppressive drugs) within the past 2 years； 7. Active infections requiring systemic treatment； 8. Known history of active tuberculosis； 9. Other significant cardiovascular diseases, including recent myocardial infarction, acute coronary syndrome, or a history of coronary artery interventions (angioplasty, stent placement, or bypass surgery) within the last 6 months; NYHA Class II-IV congestive heart failure (CHF) or a history of NYHA Class III or IV CHF； 10. Known history of human immunodeficiency virus (HIV) infection