Serial FES PET/CT to Measure Hormone Expression in Patients Undergoing Endocrine Targeted Therapy

Inclusion Criteria: * Adult, non-pregnant patients with biopsy-proven or clinically obvious primary, recurrent or metastatic breast cancer * Breast cancer from ER+ primary that is seen on other imaging tests. Tumor ER expression must have been confirmed by immunohistocytochemistry of primary tumor or recurrent disease. * At least one site of disease 1.5 cm or greater is needed to meet the spatial resolution limits of PET imaging. * Patients must have been off tamoxifen or other estrogen receptor blocking agents for at least 6 weeks and off chemotherapy for 3 weeks for the initial baseline FES. * Patients must be selected for an endocrine targeted therapy regimen for treatment of their breast cancer by the referring oncologist. Selected treatments may be part of experimental treatment protocols for which the patient would be separately consented. * Patients must be willing to undergo serial imaging procedures. * Patients must agree to allow access to clinical records regarding response to treatment and long term follow up. * Ability to understand and the willingness to sign a written informed consent document. Exclusion Criteria: * An inability to lie still for the tests * Individuals weighing more than 300 lb. (this is the weight limit of the scanner table) * Pregnant or lactating. Women of childbearing potential with either a positive or no pregnancy test at baseline are excluded. * Any other life-threatening illness (e.g. serious, uncontrolled concurrent infection or clinically significant cardiac disease - congestive heart failure, symptomatic coronary artery disease, cardiac arrhythmia not well controlled with medication). * Use of tamoxifen, faslodex, DES or any other ER blocking agent \< 6 weeks or chemotherapy \< 3 weeks prior to imaging scan. * Uncontrolled diabetes mellitus (fasting glucose \> 200 mg/dL) * Adult patients who require monitored anesthesia for PET scanning.