EMERALD: Endocrine Therapy oMission With Radiation in ER+ Breast Cancer: Assessing Quality of Life and Disease Control: a Prospective Phase II Trial

Inclusion Criteria: 1. Diagnosis of pathologically-confirmed invasive breast cancer 2. Age ≥ 60 years 3. Treatment with breast conserving surgery 4. Pathologic T stage of T1 or T2 with total tumor size ≤ 3cm including any component of DCIS that may exist in conjunction with invasive disease 5. Ki-67 . 20% and/or Oncotype 0-10 (Ultra Low) 6. Final surgical margins negative, defined as no tumor on ink. Lobular carcinoma in situ involving the final surgical margin will be disregarded. 7. Tumor must be estrogen receptor positive, progesterone receptor positive and HER2Neu non-amplified as per current College of American Pathologists guidelines 8. Clinical nodal stage cN0 9. If pathologic nodal staging is obtained, then pN0 or pN0(i+) 10. Patients have had or have a scheduled discussion with a breast medical oncologist regarding adjuvant treatment options 11. Patients who have met criteria 4.1.1-4.1.9 who never initiate endocrine therapy OR who initiate adjuvant endocrine therapy but discontinue it within 6 months of initiation \* In some clinical scenarios, patients may have a particularly small volume of tumor, making it not be feasible for all of these biomarkers to be evaluated. In this scenario, if the PI and/or Protocol Collaborators are able to document that a patient is being treated as if they have luminal A (biologically favorable) breast cancer then the patient is eligible for enrollment if all other eligibility criteria are met. Exclusion Criteria: 1. Node positive disease (N1-3) 2. Metastatic disease (M1) 3. Grade 3 disease and lymphovascular space invasion in the tumor 4. Synchronous bilateral breast cancer 5. Receipt of neoadjuvant therapy 6. Diagnosis of a collagen vascular disease associated with an increased incidence of radiation toxicities such as scleroderma or systemic lupus erythematosis 7. Diagnosis of other cancer within the prior 5 years, excluding basal or squamous cell carcinoma of the skin that has been treated 8. Patient with psychiatric illness/social situations that would limit compliance with study requirements 9. Prior radiotherapy to the index breast or recurrent cancer of the breast 10. Known carrier of a mutation associated with predisposition to breast cancer development, including BRCA1 or BRCA2