F-18 FES PET/CT in Measuring Hormone Expression in Patients With Primary, Recurrent, or Metastatic Breast Cancer 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 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, diethylstilbestrol (DES) or any other ER blocking agent \< 6 weeks or chemotherapy \< 3 weeks prior to imaging scan * Unwillingness or inability to give informed consent * Uncontrolled diabetes mellitus (fasting glucose \> 200 mg/dL) * Adult patients who require monitored anesthesia for PET scanning