16a-18F-fluoro-17b-estradiol ([18F]FES) is radioactive labeled estradiol, developed for in vivo visualization of the estrogen receptor (ER) using positron emission tomography (PET). To date, [18F]FES PET has been mainly explored as a diagnostic imaging tool to assess ER expression, thereby identifying locations of disease and their potential sensitivity to endocrine therapy, respectively. The primary aim of this project is to extend the application of [18F]FES PET as a baseline diagnostic imaging biomarker for ER expression to use it as an (early) treatment response marker. However, for such an application, visual assessment alone may not be sufficient and a more rigorous quantitative image analysis is needed. Therefore, in this project we shall first derive the optimal pharmacokinetic model for full quantitative analysis of [18F]FES uptake and, subsequently, we shall assess the validity of simplified, clinically feasible, quantitative parameters of [18F]FES uptake in 5 patients with metastatic estrogen receptor positive (ER+) breast cancer (part A). In addition, the repeatability of these simplified parameters will then be investigated in another 10 patients (part B).

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

Females only.