Inclusion Criteria:  Female ≥ 18 years of age. Histologically confirmed ER-positve breast cancer. Any pathology from a primary or metastatic breast cancer site demonstrating ER-positivity will be allowed. ECOG performance status 0 to 2 Written informed consent (and assent when applicable) obtained from subject or subject's legal representative and ability for subject to comply with the requirements of the study.  Either:  For cohort 1: Stage 2B-3C locally advanced disease and plan for CT/bone scan systemic staging or For cohort 2: Suspected recurrent disease and plan for CT/bone scan systemic staging  Exclusion Criteria:  Pregnant or breastfeeding Presence of a condition or abnormality that in the opinion of the Investigator would compromise the safety of the patient or the quality of the data. Patients who cannot undergo PET/CT scanning because of weight limits. PET/CT scanners may not be able to function with patients over 450 pounds. Selective ER Modulators (SERMs) such as tamoxifen and Selective ER Degraders (SERDs) such as fulvestrant may reduce detection of ER-positive lesions by FES. Thus, exclusion criteria specify that patients must be withdrawn from Selective ER Modifiers for at least 8 weeks and withdrawn from Selective ER Degraders for for least 24 weeks prior to performance of the FES PET/CT.