[18F]FES PET/CT in Endocrine Refractory Breast Cancer

This study will evaluate \[18F\]FES PET/CT uptake as a predictor of progression free survival in endocrine refractory recurrent or metastatic breast cancer patients starting a new therapy regimen including endocrine targeted therapy. 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. This is an observational study in that \[18F\]FES PET/CT will not be used to direct therapy decisions.