Node Negative, Invasive Breast Cancer Single Fraction IOERT

Inclusion Criteria: * Patient agrees to breast conservation therapy (segmental resection, partial mastectomy, and radiation therapy) as the treatment for their breast cancer * Patient agrees to evaluation of the axilla with sentinel lymph node biopsy * Post-menopausal women age \> 60, defined as women who have experienced no menstrual period in the past 12 months. * BRCA1 and 2 gene mutation negative, if tested. \[genetic testing is NOT required based upon personal or family history\] * Unifocal (unicentric), invasive ductal carcinoma or favorable sub-types (mucinous, tubular, colloid) \< 2.0 cm in diameter, primary T-stage of T1 (AJCC criteria) * Grade 1, 2 acceptable * Associated LCIS is allowed * Estrogen receptor (ER) status of positive * Negative margins at ink on gross pathologic examination * Patient is node-negative, defined as N0 (i-) or N0 (i+) * Patient must be deemed functionally and mentally competent to understand and sign the informed consent Exclusion Criteria: * Prior breast malignancy or other malignancy if metastatic, or with expected survival of \< 5 years * Immuno-compromised status * Pregnancy * Women with an active connective tissue disorder (i.e. scleroderma, lupus and others) * Breast cancer that involves the skin or chest wall, locally advanced breast cancer * Pure DCIS, all grades * Invasive lobular carcinoma * Evidence of lymphovascular invasion (LVI) * Invasive carcinoma with extensive intraductal component (EIC) * Neoadjuvant chemotherapy indicated * Patients with 1 or more positive lymph node determined during surgery with sentinel node and/or axillary dissection * Not eligible for breast conserving management, i.e., prior whole breast radiation therapy * Estrogen receptor negative * Her2 positive * Grade 3