A Study Comparing Cancer Imaging Approaches in People With Lobular Breast Cancer

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.