Precision 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 Trial

Inclusion Criteria * Invasive epithelial (ductal, medullary, lobular, papillary, mucinous (colloid), or tubular) histologies of the breast 3 cm or less(T1-T2cN0) in women who have not undergone surgery or neoadjuvant endocrine or chemotherapy for current breast cancer diagnosis. It is recommended that these tumors are at least 1 cm for cohort 1 and less than 1 cm for cohort 2, however this is not required. * Tumor must not involve the overlying skin based on imaging evaluation and/or clinical exam * Age \>/= 18 years old and female * If the tumor is unifocal, greatest tumor dimension is 3cm or less based on imaging. Multifocal disease may be included if the aggregate span of disease is 3 cm or less, subject to investigator evaluation and approval. * The tumor must be visible on CTor MRI scan or preferably marked with clip(s) in tumor if not visible. * Patients must have undergone standard of care breast MRI or contrast-enhanced mammography for work up to aid in tumor delineation and to rule out multicentric disease. If areas suspicious of multicentric disease are seen which are beyond the scope of SBRT, they need to have a negative biopsy to proceed with treatment. * Clinically and radiographically node negative on ultrasound of the axilla or MRI on initial workup prior to microbubble contrast assessment (for cohort 1). * Estrogen receptor positive or Progesterone receptor positive and Her2neu negative Ability to understand and the willingness to sign a written informed co…