Accurate staging is of great importance in patients with clinically locally advanced primary breast cancer (LABC, stage III) or locoregional recurrent (LRR) breast cancer for making a correct treatment plan. According to current guidelines, staging is performed with positron emission tomography (PET) using the 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG) PET tracer, combined with diagnostic computed tomography (CT). However, previous studies have shown that this technique (with the current PET tracer) might not be sufficient for accurate staging. Specifically in low grade, estrogen receptor positive (ER+) breast cancer metastases can be missed due to the low metabolic activity, leading to low uptake of [18F]FDG. Therefore, there is a clinical need to improve staging procedures. 16α-[18F]-fluoro-17β-estradiol ([18F]FES), an ER-targeted PET tracer, allows imaging of ER+ tumor lesions regardless of their metabolic activity. Patients with clinically LABC and LRR have a 25-50% risk of distant metastases. Correct identification of distant metastases allows adaptation of the treatment plan to avoid burdensome treatment with surgery, systemic and radiotherapy in order to maintain quality of life. In case of oligometastases, correct identification increases the likelihood for cure with local treatment. In the current study we will compare disease staging with [18F]FES- and [18F]FDG PET in patients with clinically LABC/LRR breast cancer. Objective: To determine whether [18F]FES PET/CT improves staging for women with clinically LABC or LRR, ER+/HER2- breast cancer as compared to standard [18F]FDG PET/CT. Study design: Multicenter prospective study with invasive measurements. Study population: 20 LABC and 20 LRR ER+/HER2- breast cancer patients. Main study parameters/endpoints: To determine the percentage of patients with a correctly changed treatment plan according to information obtained from [18F]FES PET/CT compared to [18F]FDG PET/CT at staging and at 6 months of follow-up; to determine the percentage of metastatic lesions detected and missed with [18F]FES PET/CT compared to [18F]FDG PET/CT (at staging and during follow-up). Nature and extent of the burden and risks associated with participation, benefit and group relatedness: Patients will receive an intravenous cannula for tracer injection and blood sampling, causing potentially transient discomfort at the site of the cannula insertion. Tumor biopsy will be performed from an easy accessible lesion and the most frequent complications that can occur are discomfort, bleeding and (local) infection. The risk of complications from a tumor biopsy is considered low: 0.24-1.6% and 0.11-0.48% for major complications and mortality, respectively. Radiation exposure from a [18F]FES PET and [18F]FDG PET scan usually ranges between 4-11 mSv and 7-8 mSv, respectively. Radiation exposure from a diagnostic CT scan ranges between 8-14 mSv. The total radiation burden is considered justifiable when compared to the information that can be obtained from this study, in this patient group with breast cancer. Imaging with [18F]FES PET may improve staging for patients with breast cancer as it may show tumor lesions that could not be identified with [18F]FDG PET, the current standard for staging. If this is the case, the initial treatment goal and intensity can be adjusted which can have beneficial effects for the patient.

Inclusion Criteria:

• Clinically LABC (stage III) or LRR breast cancer (all histological types) with ER+, HER2- and low grade according to Bloom Richardson criteria (grade 1-2)
• Females aged 18 years or older at screening
• Eastern Cooperative Oncology Group Performance Status (ECOG PS) of 0-2
• Candidates for treatment with curative intent (patients are also allowed for inclusion in the current study if they have undergone recent surgery (<6 weeks) for current breast cancer and require staging because of unexpected stage III disease)
• In case [18F]FDG PET/CT has already been performed, patients can be included <21 days after this scan
• 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 non-melanoma skin cancer
• Undergoing treatment for current breast cancer such as (neo)adjuvant chemotherapy, hormonal therapy (only in case of Tamoxifen), radiotherapy 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