To evaluate pathologic and radiologic responses to pre-operative stereotactic ablative body radiotherapy (SABR) in early-stage breast carcinoma as well as treatment-related toxicity and cosmesis outcomes. To test hypothesis that pre-operative SABR will produce at least 10% probability of pathologic complete response. This investigation was conducted as a phase-2 prospective single-institution trial. Study population included women over 50 years of age with low-risk breast cancer defined as requiring all the following: unifocal tumor less than 2cm in size, ER+, Her-2 non-amplified, clinically node-negative, and non-lobular histology. Eligible patients underwent pre-operative SABR to 28.5 Gy in 3 fractions of 9.5 Gy/fraction followed by breast conserving surgery and sentinel lymph node (SLN) evaluation 6-8 weeks later. Diagnostic MRI was registered to simulation CT in all patients to define target volumes. The GTV was expanded by 15mm for CTV and 3mm was added for PTV. All patients were simulated and treated in the prone position. Primary endpoint was pathologic response. Secondary endpoints included radiologic responses on MRI, toxicity and cosmesis. Twenty patients were enrolled and treated. No pathologic complete responses were observed. With median follow-up of 13.5 (4-24) months, no local recurrences have been observed. Four patients were incidentally node-positive following SLN evaluation, two micro-metastases and two macro-metastases, and underwent post-SABR whole-breast radiotherapy. In patients who only had SABR, 15% (3) and 5% (1) developed late grade 2 or 3 soft-tissue toxicity, respectively. Of the 4 node-positive cases who received both SABR and whole-breast radiotherapy, two developed Gr 3 late soft-tissue toxicity. Pre-operative SABR failed to achieve pathologic complete response at 6-8 weeks following treatment although was well tolerated and improved convenience for most patients. Significant risk remains even in this low-risk cohort for positive nodes; 20% in our series. The resultant addition of whole breast radiotherapy in these cases may increase risk of grade 3 late toxicity.