FES PET/CT in Predicting Response in Patients With Newly Diagnosed Metastatic Breast Cancer Receiving Endocrine Therapy

Inclusion Criteria: * Capable and willing to provide informed consent * Women must not be pregnant or breast-feeding. All females of childbearing potential must have a blood test or urine study within 7 days prior to FES PET/CT scan and \[18F\]-fluorodeoxyglucose (FDG)-PET/CT scan to rule out pregnancy; a female of childbearing potential is any woman, 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) * Women of childbearing potential and sexually active males must 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 postmenopausal woman, man, or premenopausal woman for whom standard endocrine therapy alone (tamoxifen, aromatase inhibitor \[AI\], with or without ovarian suppression or fulvestrant) 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 Eastern Cooperative Oncology Group (ECOG) performance status of 0-3 (restricted to ECOG performance status \[PS\] 0-2 if age \> 70 years) * Patient must NOT have a history of allergic reaction attributable to comp…