Radiation Omission in Patients With HER2 Overexpressing Tumors With Pathologic Complete Response

Inclusion Criteria: * Female * Age ≥ 40 years * Patients must have a tissue diagnosis of HER2+ breast cancer * Patients must have documented clinical T1-3, N0, M0 disease prior to receiving neo-adjuvant chemotherapy * Patients must have undergone neo-adjuvant chemotherapy and targeted HER2 therapy prior to surgery and plan for completion of adjuvant systemic therapy as directed by their medical oncologist * Patients must have completed partial mastectomy/lumpectomy and sentinel node biopsy or axillary node dissection. * Patient must have documented pathologic complete response (defined as no residual invasive or in situ disease in the breast or axilla, including free of isolated tumor cells or micrometastasis) Exclusion Criteria: * Breastfeeding * Prior history of breast cancer (invasive or ductal carcinoma in-situ) in either breast * Prior adjuvant radiation therapy * Patients with diagnosis of inflammatory breast cancer * Patients with known BRCA mutation or other known breast cancer related deleterious mutations