Dynamic and Test-retest Whole Body [18F]FES PET Imaging in Patients With Metastatic ER+ Breast Cancer

Inclusion Criteria: * Histologically proven metastatic ER+ (\>10% positive stained cells using immunohistochemistry) breast cancer on the latest biopsy * Postmenopausal females aged 18 years or older at screening. Postmenopausal status is defined as one of the following: 1. age ≥60 years 2. age \<60 years and amenorrhea for \>12 months in the absence of interfering hormonal therapies (such as LH-RH agonists and ER-antagonists) 3. patient age \<60 years using LH-RH agonists should continue LH- RH-agonists until after the PET procedures 4. previous bilateral oophorectomy or medically confirmed ovarian failure * \[18F\]FDG PET, CT and/or a bone scan should be performed as part of routine clinical staging (≤4 weeks prior to screening) * Patients should have metastases in the scanning field of view, all located outside of the liver * Eastern Cooperative Oncology Group Performance Status (ECOG PS) of 0-2 * Estimated glomerular filtration rate (eGFR) ≥30 ml/min * Written and signed informed consent Exclusion Criteria: * History with another cancer within the last 5 years, except cancer treated with curative intent and no evidence of disease as judged by the treating physician * Use of selective estrogen receptor modulators (SERMs) or downregulators (SERDs) for current breast cancer such as Tamoxifen/Fulvestrant (≤5 weeks prior to screening) or investigational drug therapy * Pregnancy or lactating women * Any medical, psychological or social condition that may interfere with the subject's safety and participation in the study, will lead to exclusion from this study