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 consent. 10. Women of child-bearing potential must agree to use adequate contraception (hormonal or barrier method of birth control) prior to the start of study and for the duration of radiation therapy as well as the endocrine therapy. Should a woman become pregnant or suspect she is pregnant while participating in this study, she should inform her treating physician immediately A female of child-bearing potential is any woman (regardless of sexual orientation, having undergone a tubal ligation, or remaining celibate by choice) who meets the following criteria: * Has not undergone a hysterectomy or bilateral oophorectomy; or * Has not been naturally postmenopausal for at least 12 consecutive months (i.e., has had menses at any time in the preceding 12 consecutive months * For Cohort 1: If patient has had a prior biopsy clip placed in the lymph node deemed the sentinel lymph node at time of microbubble CEUS, it is up to investigator if additional biopsy and clip placement will be obtained. Exclusion Criteria * Multi-centric disease * Prior Radiation to the involved breast * Aggregate or single tumor Size \>3cm * Patients who are pregnant or lactating due to the potential exposure to the fetus to radiation therapy and unknown effects of radiation therapy to lactating females * Prior ipsilateral breast cancer * Patients with active lupus or scleroderma * Patients unable to have MRI or contrast-enhanced mammography per physician assessment. * For Cohort 1: If patient has a positive lymph node at time of microbubble contrast enhanced ultrasound, they will be removed from the study. Only cN0 patients to be treated on this study. Healthy Volunteer Inclusion Criteria * Ability to understand and the willingness to sign a written informed consent. * Age \>/= 18 years old and female * Without current diagnosis of breast cancer Healthy Volunteer Exclusion Criteria * Contraindication ot MRI * Volunteers who are pregnant