RecruitingPhase 2Radiation therapyPrecision Medicine in Action: Phase II Trial of Response Adaptive Ablative Pre-operative SPBI (RAPS) and Non-operative Sentinel Lymph Node Biopsy in Patients With Early-stage ER+ Breast Cancer: RAPS Trial1\. Efficacy of PULSAR preoperative radiation 2. Evaluate potential of microbubble CEUS as an alternative to operative SLNBx 3. Evaluate potential of OA to evaluate treatment response of pre-operative radiation on the tumor