The purpose of this study is to look at how effective neoadjuvant (before surgery) endocrine therapy (NET) is in participants with invasive lobular carcinoma (ILC) who have breast-conserving surgery (BCS). The main purpose of the study is to see if NET reduces the chance of having cancer cells at the edges of tissue removed during surgery (positive margins).

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.

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 following one year after publication and for up to 36 months later. 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.