Patients with recurrent or metastatic ER+ breast cancer, who have failed prior endocrine therapy may be eligible for this study.  Patients may participate in this study, if they are at least 18 years of age, most participants will be receiving care at the clinical practices of the University of Pennsylvania.

Positron emission tomography (PET/CT) imaging will be used to evaluate estrogen receptor (ER) activity, in sites of metastatic disease, using the investigational radiotracer [¹⁸F]fluoroestradiol (FES).  Imaging will occur prior to starting new therapy.  Some patients may also undergo a second FES PET/CT scan, at the time of suspected progression of disease to compare for changes in FES uptake measures.

Patients may undergo primary tumor or metastatic biopsy, as part of their clinical care.  Patients may also be enrolled in a separate breast cancer biopsy protocol entitled “Metastatic Markers of Recurrent Tumor Phenotype for Breast Cancer” (METAMORPH).  We will ask permission to access tissue collected as part of clinical biopsy or METAMORPH, if available.  If the patient gives permission, archived tissue may be accessed for the purposes of this study to perform additional experimental pathology assays.