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.

Inclusion Criteria:

• 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
• ECOG Performance Status Score of 0, 1 or 2.
• Patient can have received up to 2 lines of therapy in the metastatic setting
• Life expectancy of at least 12 months

Exclusion Criteria:

• 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.

Patients will be recruited from breast medical oncology clinics, with a concerted effort to recruit a diverse group of patients. Recruitment will be led by Co-PI Komal Jhaveri, MD, Assistant Attending, Breast Medicine Service. Eligible patients will be approached to enroll in the protocol and provide written informed consent. Potential research subjects will be identified by a member of the patient's treatment team, the protocol investigator, or research team at MSK. If the investigator is a member of the treatment team, s/he will screen their patient's medical records for suitable research study participants and discuss the study and their potential for enrolling in the research study.

Memorial Sloan Kettering Cancer Center supports the international committee of medical journal editors (ICMJE) and the ethical obligation of responsible sharing of data from clinical trials. The protocol summary, a statistical summary, and informed consent form will be made available on clinicaltrials.gov when required as a condition of Federal awards, other agreements supporting the research and/or as otherwise required. Requests for deidentified individual participant data can be made beginning 12 months after publication and for up to 36 months post publication. Deidentified individual participant data reported in the manuscript will be shared under the terms of a Data Use Agreement and may only be used for approved proposals. Requests may be made to: crdatashare@mskcc.org.