In this study, positron emission tomography (PET/CT) imaging will be used to evaluate estrogen receptor (ER) activity in sites of metastatic disease using the investigational radiotracer [18F]fluoroestradiol (FES).

This study will evaluate [18F]FES PET/CT uptake as a predictor of progression free survival in endocrine refractory recurrent or metastatic breast cancer patients starting a new therapy regimen including endocrine targeted therapy. Imaging will occur prior to starting new therapy. Some patients may also undergo a second FES PET/CT scan at the time of suspected progression of disease to compare for changes in FES uptake measures. This is an observational study in that [18F]FES PET/CT will not be used to direct therapy decisions.

Inclusion Criteria:

1. At least 18 years of age
2. Recurrent or metastatic cancer that is of known or suspected breast origin - may be biopsy proven or identified on standard imaging (e.g. CT, bone scan, MRI, FDG PET/CT)
3. History of ER+ pathology (ER+ may be confirmed from surgery or biopsy of primary breast cancer or lymph nodes, and/or surgery or biopsy of a metastatic site, metastatic biopsy is not required)
4. At least one site of disease outside of the liver that is seen on standard imaging (e.g. CT, bone scan, MRI, FDG PET/CT); patients with measurable or nonmeasurable disease are allowed.
5. History of progression or recurrence of disease while on an endocrine targeted therapy containing regimen as assessed by medical record review of breast cancer history at screening
6. Participants must be informed of the investigational nature of this study and be willing to provide written informed consent and participate in this study in accordance with institutional and federal guidelines prior to study-specific procedures

Exclusion Criteria:

1. Females who are pregnant at the time of screening will not be eligible for this study, urine pregnancy test will be performed at screening in women of child-bearing potential.
2. Inability to tolerate imaging procedures in the opinion of an investigator or treating physician
3. Serious or unstable medical or psychological conditions that, in the opinion of the investigator, would compromise the subject's safety or successful participation in the study.
4. History of HER2/neu positive cancer (IHC 3+ and/or FISH positive) as assessed by medical record review at screening