This is a prospective multisite exploratory study for women ≥ 65 years of age with early stage estrogen receptor positive (ER+) breast cancer. These individuals will be treated with 3 months of pre-operative endocrine therapy (pre-ET) with assessment of tolerance to the endocrine therapy by patient reported outcome (PRO) measures (patient surveys).

This single arm pilot study evaluates the use of pre-operative endocrine therapy in women with early stage ER+ breast cancer who are 65 years or older and are scheduled to have breast conservation surgery (BCS). The purpose of this study is to see if tolerance of pre-operative endocrine therapy helps inform decisions on whether or not to omit radiation. Participants will be treated with the novel approach of 3 months of pre-operative endocrine therapy, so that the participants and providers will have some indication of tolerance prior to making a decision about radiation omission. Tolerance of the endocrine therapy will be assessed by patient reported outcome (PRO) measures during neoadjuvant and adjuvant endocrine therapy treatment periods. Breast cancer participants and their surgical oncologists will be asked their preference regarding radiation therapy before starting endocrine therapy and again pre-operatively. They will make a determination regarding radiation therapy as per standard care. The hypothesis is that knowledge of tolerance of endocrine therapy will influence the decision by the patient and the surgical oncologist whether or not to elect radiation omission.

Inclusion Criteria (Summary):

• ECOG performance status 0-2
• Diagnosed with anatomic stage I, ER positive, PR positive or negative, and HER2 non amplified invasive breast cancer and clinically negative nodes; any invasive breast cancer histologic subtype may be enrolled
• Tumor size ≤ 2 cm
• Patient has elected BCS as surgical choice
• Eligible to receive tamoxifen or an aromatase inhibitor
• Ability to take oral medication and be willing to adhere to the endocrine therapy for the 3 month period prior to BCS

Exclusion Criteria (Summary):

• Prior or current use of endocrine therapy for breast cancer
• History of ipsilateral breast radiation
• Pregnancy or lactation
• Has a known additional malignancy that is progressing or requires active treatment. Exceptions include basal cell carcinoma of the skin or squamous cell carcinoma of the skin that has undergone potentially curative therapy or in situ cervical cancer.
• Current or planned use of a strong CYP2D6 inhibitor (e.g. Fluoxetine, Paroxetine) and is not able to receive an endocrine therapy agent that does not use the CYP2D6 pathway