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.