Initial CDK 4/6 Inhibitor Plus Endocrine Therapy Versus Initial Chemotherapy in HR Positive, HER2 Negative Unresectable or Metastatic Breast Cancer

Inclusion Criteria: 1. age \>= 18 years old; 2. Patients who were diagnosed as HR Positive, HER2 Negative recurrent unresectable (local or regional) or stage IV (M1) breast cancer between June 2020 to October 2023; 3. Received CDK4/6 inhibitor plus endocrine treatment or chemotherapy as first-line therapy for at least one cycle; 4. Complete medical history was available. Exclusion Criteria: 1\. Medical history was incomplete.