Inclusion Criteria:

• Age ≥ 18 years.
• Capable and willing to provide informed consent.
• Patients must not be pregnant or breast-feeding due to harmful risks of PET/CT scans to the fetus. All females of childbearing potential must have a blood test or urine study within 7 days prior to FES PET/CT scan and FDG-PET/CT scan to rule out pregnancy. A patient of childbearing potential is anyone, regardless of sexual orientation or whether they have undergone tubal ligation, who meets the following criteria:
  • has not undergone a hysterectomy or bilateral oophorectomy; or
  • has not been naturally postmenopausal for at least 24 consecutive months (i.e., has had menses at any time in the preceding 24 consecutive months).
  • Female of child bearing potential? ______ (Yes or No) Date of blood test or urine study: ___________
• Patients of childbearing potential and sexually active men must indicate their agreement to use an accepted and effective method of contraception or to abstain from sexual intercourse for the duration of their participation in the study.
• Patient is a premenopausal woman, postmenopausal woman, or man for whom endocrine therapy (tamoxifen, aromatase inhibitor [AI], with or without ovarian suppression or fulvestrant), with or without a CDK4/6 inhibitor, is planned after FES-PET/CT is completed.
• Medically stable as judged by patient’s physician
• Life expectancy must be estimated by patient’s physician at > 6 months.
• Patients must have an ECOG performance status of 0-3 (restricted to ECOG PS 0-2 if age > 70 years).
• Patient must NOT have a history of allergic reaction attributable to compounds of similar chemical or biologic composition to 18F-FES.
• Patient must NOT be in liver failure as judged by the patient’s physician.
• Patient must have ER+ breast cancer histologically confirmed either from a metastatic biopsy or from a primary breast tumor. If biopsy is NOT available for metastatic disease, then metastatic disease must be documented by two imaging methods. Correlative radiology studies must be within 6 weeks prior to FESPET/CT, and at least one method of correlative imaging is required for all participants. Preferred modality of imaging is FDG-PET/CT. Patients with effusion only disease or disease only in the liver are not eligible for the study.
• The pathology report AND either (1) tumor tissue (blocks or unstained slides) OR (2) a photomicrograph of the ER IHC slides from at least one site of metastatic disease and/or from primary breast cancer must be available for central review and analysis.
  • NOTE: If photomicrographs are submitted, the submission of H&E, PR and Ki67 IHC’s, if performed, are also to be submitted. Photomicrograph submissions are to be submitted via TRIAD.
• Patient must NOT have human epidermal growth factor-2 positive (HER2+) metastatic disease.
• Patient must NOT be planning to receive everolimus nor HER2 directed therapy in addition to endocrine therapy.
• Patient must NOT have received prior endocrine therapy for metastatic disease (i.e., must be first-line endocrine therapy for metastatic disease).
• Patient is not now, and never has received adjuvant endocrine therapy OR Patient is currently receiving or has received adjuvant endocrine therapy in the past, AND adjuvant endocrine therapy was initiated > 2 years prior to diagnosis of metastatic disease.
  • NOTE: Patients who developed metastatic disease while still receiving adjuvant endocrine therapy must have a planned change in the type of endocrine agent used for subsequent metastatic disease treatment.
• Patient is not receiving blocking adjuvant therapy (such as toremifene or tamoxifen) OR Patient is receiving blocking adjuvant therapy, but will stop this therapy a minimum of 60 days prior to FES-PET/CT while still complying with the study timeline.
• Patient must NOT have a history of > 1 line of administered chemotherapy for metastatic disease and must be off chemotherapy for a minimum of 2 weeks. Prior chemotherapy in the adjuvant setting is allowed.
• Disease must be present in at least one non-liver site Diease must be measurable (by RECIST 1.1 criteria) and be 1.5 cm or greater in longest dimension OR disease can be non-measurable but must be 1.5 cm or greater in longest dimension on functional imaging (FDG-PET/CT).
• Patient must be able to lie still for a 20-30 minute PET/CT scan.
• Patient must NOT weigh more than the maximum weight limit for the table for the PET/CT scanner at the institution where the study is being performed.
• The patient is participating in the trial at an institution which has agreed to perform the imaging research studies, completed the ECOG-ACRIN defined scanner qualification procedures and received ECOG-ACRIN (or current ACRIN) approval approval.