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:  age ≥60 years age <60 years and amenorrhea for >12 months in the absence of interfering hormonal therapies (such as LH-RH agonists and ER-antagonists) patient age <60 years using LH-RH agonists should continue LH- RH-agonists until after the PET procedures 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