To investigate parameters based on 18F-FES-PET/CT so as to estimate the outcome of palbociclib combined with endocrine therapy in patients with HR+/HER2-MBC.

Inclusion Criteria:

1. Female patients aged 18-70 years.
2. Metastatic breast cancer included unresectable locally advanced breast cancer, de novo stage IV breast cancer, and recurrent metastatic breast cancer.
3. HR+ was defined as estrogen receptor- or progesterone receptor-positive status by immunohistochemistry.
4. HER2 status was determined by immunohistochemistry or fluorescence in situ hybridization.
5. Palbociclib treatment of metastatic breast cancer for at least one cycle.
6. Available medical history.

Exclusion Criteria:

1.Incomplete medical history.

Patients diagnosed with breast cancer with confirmed metastasis, regardless of being de novo diagnosed or progressed from a non-metastatic stage.