The purpose of this study to compare two types of cancer imaging scans to see which is better at detecting and monitoring metastatic ILC. This study will compare PET/CT (positron emission tomography/computed tomography) scans using the radiotracer fluorine 18 (18F)-fluoroestradiol (FES) with a standard imaging approach for detecting and monitoring ILC, PET/CT scans using the radiotracer 18F-fluorodeoxyglucose (FDG). These scans will be referred to as FES PET/CT scans and FDG PET/CT scans.
Adult patient > 18 years old
Biopsy-proven metastatic ILC
Confirmed ER+ breast cancer from either primary tumor or metastatic biopsy (ER receptor percent staining ≥ 10% of tumor cells by IHC)
HER2-negative per ASCO/CAP
Low ER expression defined as ER <10% of tumor cells by IHC
Patients who have been on adjuvant ER-blocking endocrine therapy with SERM or SERD without an 8- and 24-weeks washout, respectively. (The use of aromatase inhibitors or ovarian suppression is permitted given that they do not block estrogen receptors).
Pregnancy or lactation or parturient or nursing mother
Patient presents with any other clinically active, serious, life-threatening disease, medical, or psychiatric condition, and/or who has a life expectancy of <12 months, or for whom study participation may compromise their management; and/or a patient who the investigator judges to be unsuitable for participation in the study for any reason.