Positive ER Expression Assessed by 18F-FES PET/CT in the MBC with ER-negative Primary Tumor

Inclusion Criteria: * Patients diagnosed with breast cancer (according to International Classification of Diseases 10th Revision) with confirmed metastasis, regardless of being de novo diagnosed or progressed from a nonmetastatic stage. * MBC patients who underwent 18F-FES PET/CT at the Fudan University Shanghai Cancer Center between 2012 and 2023 * Patients diagnosed with primary ER-negative tumor and who were yet to receive any systemic therapy during the advanced stage Exclusion Criteria: Patients with incomplete medical records and those diagnosed with secondary primary tumors