The standard of care imaging of breast cancer metastases recommended by the National Comprehensive Cancer Network (NCCN), CT of the chest/abdomen/pelvis and bone scan, may be suboptimal. 18F-fluoroestradiol (FES) is a novel PET tracer designed to detect estrogen receptors, which are often expressed in breast cancers. FES PET/CT may provide improved evaluation of metastases in patients with ER-positive breast cancer.

This will be a phase II clinical trial to assess the clinical value of FES for staging and detection of disease recurrence in patients with ER-positive breast cancer, in direct comparison to the National Comprehensive Cancer Network's (NCCN) recommended standard-of-care using CT of the chest/abdomen/pelvis and bone scan [2].

Two cohorts will be evaluated:

1. Patients with advanced local breast cancer that will undergo standard of care imaging (CT + bone scan) to evaluate for unsuspected distant metastases.
2. Patients with suspicion for breast cancer recurrence that will undergo standard of care imaging (CT + bone scan) to evaluate for disease recurrence.

In both cohorts, FES PET/CT will be obtained, and compared with standard of care imaging. Whether on standard of care imaging or the research FES PET/CT, lesions suspicious for unsuspected distant metastases (cohort 1) or the site of disease recurrence (cohort 2) will be selected for biopsy, as is clinically standard of care to confirm disease that would alter patient management. Pathology will be used as the reference standard for confirming malignancy.

Inclusion Criteria:

1. Female ≥ 18 years of age.

2. Histologically confirmed ER-positve breast cancer. Any pathology from a primary or metastatic breast cancer site demonstrating ER-positivity will be allowed.

3. ECOG performance status 0 to 2

4. Written informed consent (and assent when applicable) obtained from subject or subject's legal representative and ability for subject to comply with the requirements of the study.

5. Either:

   1. For cohort 1: Stage 2B-3C locally advanced disease and plan for CT/bone scan systemic staging or
   2. For cohort 2: Suspected recurrent disease and plan for CT/bone scan systemic staging

Exclusion Criteria:

1. Pregnant or breastfeeding
2. Presence of a condition or abnormality that in the opinion of the Investigator would compromise the safety of the patient or the quality of the data.
3. Patients who cannot undergo PET/CT scanning because of weight limits. PET/CT scanners may not be able to function with patients over 450 pounds.
4. Selective ER Modulators (SERMs) such as tamoxifen and Selective ER Degraders (SERDs) such as fulvestrant may reduce detection of ER-positive lesions by FES. Thus, exclusion criteria specify that patients must be withdrawn from Selective ER Modifiers for at least 8 weeks and withdrawn from Selective ER Degraders for for least 24 weeks prior to performance of the FES PET/CT.