A Study of the Impact of Endocrine Therapy on Surgical Outcomes in People With Lobular Breast Cancer

Inclusion Criteria: * Written informed consent by participant or legally authorized representative * Postmenopausal women aged ≥50 years with biopsy-proven cT2-T3 N0-1 ILC who opt to undergo and are medically fit to undergo BCS at enrollment. * Tumors of the ER+/HER2- subtype, defined as: 1. ER+: Positive for ER staining as indicated by ≥10% immunoreactive tumor nuclei. 2. HER2-: Immunohistochemistry assay demonstrating no or faint staining in ≤10% of tumor cells (IHC 0 or 1+) or negative by dual probe in situ hybridization assay. Exclusion Criteria: * Patients with prior ipsilateral breast cancer. * Patients with advanced regional disease (cN2/cN3) or stage 4 disease. * Patients who would benefit from neoadjuvant chemotherapy, per the treating medical oncologist. * Patients who are not candidates for definitive breast surgery (inoperable or stage 4 disease). * Patients with gadolinium allergy, precluding the use of breast MRI.