Active Surveillance in Older Women With ER+ Breast Cancer

Inclusion Criteria: * Women aged 70 years or older with a diagnosis of invasive breast cancer who have not undergone surgical resection of the primary invasive tumor and/or axillary lymph nodes, have not undertaken any systemic therapy, and have not received radiation therapy as a treatment for this diagnosis. Patients with invasive cancer that is identified after excisional biopsy for atypia are included. * Tumors must have been identified through mammographic screening. * Tumor is less than or equal to 2cm (T1a through T1c) in maximum dimension based on diagnostic ultrasound, or if not visible on ultrasound, then on mammogram. Must have clinically and radiographically node negative disease. Inclusion criteria for different tumor sizes is as follows: * cT1a or cT1b (≤ 1cm): Nottingham Grade I or II allowed entry. * cT1c (\>1-2cm): only Nottingham Grade I allowed entry. * Breast cancer must be ER positive and HER2 negative according to the definition below, as assessed by local pathology. * ER is considered positive if there are ≥ 60% positive tumor nuclei in the samples. * HER2 negativity is defined per the current ASCO/CAP Clinical Practice Guideline. * Patients with cognitive impairment are eligible provided that a legal surrogate is able to sign informed consent for study participation. * Archival tissue will be submitted for all participants. Tissue must be confirmed available prior to registration. Exclusion Criteria: * Prior anti-cancer therapy (e.g., endocrine therapy, chemotherapy, radiation therapy, or investigational therapy) for the current breast cancer diagnosis. * Current breast cancer diagnosis that is deemed a recurrence, at the discretion of the treating investigator. * Multifocal or multicentric disease. * Patients with a history of contralateral DCIS, or ipsilateral or contralateral LCIS are not eligible. Patients with a history of any ipsilateral breast radiation are not eligible. * Diagnosis of inflammatory breast cancer (T4d). * Male breast cancer. * Any concurrent severe and uncontrolled medical condition that, in the treating clinician's opinion, would pose unacceptable safety risks or compromise compliance with the protocol. Such conditions could include: impairment of gastrointestinal tract function or gastrointestinal disease that may significantly alter the absorption of oral medications (uncontrolled Crohn disease or ulcerative colitis, uncontrolled chronic nausea, vomiting diarrhea, malabsorption, or small bowel resection); severe liver impairment (Child-Pugh Class C).