Pre-Operative Window of ET to Inform RT Decisions (POWER II)

Inclusion Criteria (summary): * Diagnosis of ER+, PR +/-, and HER2- non amplified invasive breast cancer and clinically negative nodes * ECOG performance status 0-2 * Females, aged ≥ 65 years * Patient is eligible for BCS and opted for BCS * Patient is a candidate for radiation therapy * Patient is a candidate for endocrine therapy (tamoxifen or an aromatase inhibitor) * Ability to take oral medication and be willing to adhere to endocrine therapy for the 3-month period prior to BCS * Agreement to adhere to Lifestyle Considerations (details in protocol) throughout study duration * Provision of signed and dated informed consent form * Stated willingness to comply with all study procedures and availability for the duration of the study Exclusion Criteria: * Bilateral synchronous breast cancer * Multicentric disease * Prior use of Tamoxifen or aromatase inhibitors * History of ipsilateral breast radiation therapy * Has a known additional malignancy that is progressing and/or requires active treatment with cytotoxic chemotherapy or radiation therapy. Malignancies deemed stable and low risk for complication per investigator's judgment may be allowed after discussion with multi-site PI. * Current or planned use of a strong CYP2D6 inhibitor (e.g., Fluvoxamine, Paroxetine) and is not able to receive an endocrine therapy agent that does not use the CYP2D6 pathway.