Fluorestradiol (FES) PET/CT for Imaging Estrogen Receptor Status

Inclusion Criteria: * Patients with histologic/immunochemical proof of ER+ primary or metastatic malignancy (positive staining in ≥ 1% of cells by immunohistochemistry). * Patients who are to be treated with clinically approved or experimental regimens where ER has an important role * ECOG performance status of 0-2. * Patients must provide written informed consent Exclusion Criteria: * Age \<18 years * Patients who cannot undergo PET/CT scanning (i.e. because of weight limits, claustrophobia) * Pregnancy or lactation * Total serum bilirubin \> 1.5 times upper limit of normal (abnormal hepatic metabolism may interfere with FES hepatic excretion). Increased serum bilirubin due to Gilbert's syndrome is permitted. * Serum creatinine \> 1.5 times upper limit of normal.