This study is evaluating whether position emission tomography (PET) imaging of the brain using 18F-FES can better detect and better provide targeted treatment of brain metastases in patients with endocrin resistant (ER)-positive breast cancer.  

Although PET imaging using 18F-FES is approved by the U.S. Food and Drug Administration (FDA) for the detection of ER-positive lesions in patients with recurrent or metastatic breast cancer, there is limited information about whether dedicated brain PET/CT imaging with this radiotracer can detect and help guide treatment of brain metastases. 

Participants will be asked to undergo a PET/CT scan of their head using 18F- FES for research purposes. This scan will be completed prior to the planned standard of care radiation therapy. 

Participants will be followed for a period of up to 12 months with routine standard-of-care clinical follow-up with their clinical care team. Information from medical records relating to radiation treatment, and follow-up clinical and magnetic resonance imaging (MRI) assessments will be extracted from participants' medical records from their standard of care visits and will be used for research purposes.