[18F]Fluoroestradiol (FES) PET/CT Imaging to Evaluate in Vivo Estrogen Receptor Activity in Endocrine Refractory Recurrent or Metastatic Breast Cancer

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 provide
written informed consent 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