Test - Retest Reproducibility of 18F Fluoroestradiol (FES) PET

Inclusion Criteria:

* Patients must be over 18 years old and capable and willing to provide informed consent.
* Patients of childbearing potential must have a negative urine or serum pregnancy test within 7 days prior to PET/CT imaging per institution's standard of care;
* A female of child-bearing potential is any woman (regardless of sexual orientation, having undergone a tubal ligation, or remaining celibate by choice) who meets the following criteria;
* Has not undergone a hysterectomy or bilateral oophorectomy; or
* Has not been naturally postmenopausal for at least 12 consecutive months (i.e., has had menses at any time in the preceding 12 consecutive months).
* Medically stable as judged by patient's physician.
* Life expectancy must be estimated at \> 6 months.
* Patients must have an ECOG performance status of 0-3 (restricted to ECOG PS 0-2 if age \>70 years).
* Patients with known allergic or hypersensitivity reactions to previously administered radiopharmaceuticals of similar chemical or biologic composition to FES are NOT eligible.
* Patients with liver failure are NOT eligible.
* Patient must NOT be breast-feeding.
* Histologically confirmed ER+ breast cancer either from a metastatic biopsy or from a primary breast tumor with imaging evidence of metastatic disease. The pathology report and either (1) tumor tissue (blocks or unstained slides) or (2) a photomicrograph of the ER IHC slide from at least one site of metastatic disease
* No prior endocrine therapy for metastatic disease is allowed (i.e. must be first-line endocrine therapy for metastatic disease). However, a history of adjuvant endocrine therapy is allowed, as long as the date of diagnosis of metastatic disease is \> 2 years following initiation of adjuvant endocrine therapy. Patients who develop metastatic disease while still receiving adjuvant endocrine therapy must have a change in the type of endocrine agent used for subsequent metastatic disease treatment. Patients on blocking adjuvant therapy (with a blocking agent such as toremifene or tamoxifen) must be off the agents for a minimum of 60 days to allow for adequate uptake of FES
* Patients with human epidermal growth factor-2 positive (HER2+) metastatic tumors are NOT eligible
* Postmenopausal women, men, or premenopausal women 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
* Disease may be measurable (by RECIST 1.1 criteria) or non-measurable but must be present in at least one non-liver site, 1.5 cm or greater and visualized on PET/CT with \[18F\]-fluorodeoxyglucose (FDG). Patients with effusion only disease or disease only in the liver are not eligible for the study
* Patient must be able to lie still for a 20 to 30 minute PET/CT scan.

Exclusion Criteria:

. Patients not meeting the inclusion criteria.