[18F]Fluoroestradiol-PET/CT Companion Imaging Study to the FORESEE Trial

Inclusion Criteria: * Adults aged 18 years or greater. * All patients or legal guardians are willing and able to sign written informed consent and HIPAA authorization in accordance with local and institutional guidelines. * Enrolled on the FORESEE trial. * Biopsy proven estrogen receptor positive breast cancer. * Patient deemed refractory to all combinations of hormonal therapies by FORESEE trial investigators. Exclusion Criteria: * Patients with known allergic or hypersensitivity reactions to previously administered radiopharmaceuticals. Patients with significant drug or other allergies or autoimmune diseases may be enrolled at the Investigator's discretion. * Patients who require monitored anesthesia for PET/CT scanning. * Patients who are too claustrophobic to undergo PET/CT scanning. * Patients who are pregnant or currently breast feeding. * Any patient that is medically unstable defined as a patient requiring inpatient hospitalization or needing evaluation at an acute care or urgent care facility at the time of imaging.