Serial FES PET/CT to Measure Hormone Expression in Patients Undergoing Endocrine Targeted Therapy

OUTLINE: Patients undergo F-18 FES PET/CT scan at baseline. Patients also undergo F-18 FES PET/CT and FDG PET/CT between 1-12 weeks after starting therapy, and then 1-12 weeks after the second FES PET/CT scan. Repeat FDG PET may be omitted in patients on selective estrogen receptor degrader. After completion of study, patients are followed up for up to 20 years.