This clinical trial studies use of F-18 16 alpha-fluoroestradiol ([F-18] FES) positron emission tomography (PET)/computed tomography (CT) in measuring tumor hormone receptor expression in patients undergoing endocrine-targeted therapy for newly diagnosed breast cancer or breast cancer that has come back or spread to other places in the body. Comparing results of diagnostic procedures done before, during, and after hormone therapy may help measure a patient's response to treatment.

OUTLINE:

Patients undergo F-18 FES PET/CT scan at baseline. Patients also undergo F-18 FES PET/CT and FDG PET/CT between 1-12 weeks after starting therapy, and then 1-12 weeks after the second FES PET/CT scan. Repeat FDG PET may be omitted in patients on selective estrogen receptor degrader.

After completion of study, patients are followed up for up to 20 years.

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.