A multi-center, real world study to evaluate the clinical outcomes between initial CDK 4/6 Inhibitor plus endocrine therapy and 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.

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.