• Biopsy-proven or clinically obvious breast cancer
• Positive for estrogen receptor (ER) expression in the primary tumor and/or metastatic site.
• At least one site of disease ≥ 1.5 cm
• Menopausal status documented
• Willing to undergo imaging procedures. Select patients will return for serial imaging
• Must have been off tamoxifen or other estrogen receptor blocking agents for at least 6 weeks and off chemotherapy for 3 weeks.
• Plan to have hormonal therapy regimen for treatment of their breast cancer
• Pregnant or lactating
• Inability to tolerate scanning (e.g. - claustrophobia, severe pain).
• Uncontrolled diabetes mellitus (fasting glucose > 200 mg/dL).
• Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements.
• Individuals weighing more than 300 lb. (this is the weight limit of the scanner table)
• Use of tamoxifen, faslodex, DES or any other ER blocking agent < 6 weeks or chemotherapy < 3 weeks prior to imaging scan.