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: * 1\. 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. For cohort 1, it is highly recommended those tumors are at least 1 cm. 2\. Tumor must not involve the overlying skin based on imaging evaluation and/or clinical exam 3. Age \>/= 18 years old and female 4. Greatest Tumor dimension is 3cm or less based on US. MRI measurements can be included only if performed BEFORE the biopsy 5. Tumor must be unifocal 6. The tumor must be visible on CT scan and/or preferably marked with clip(s) in tumor if not visible. At least one clip should be placed in or around tumor prior to enrollment 7. Patients must undergo an MRI for work up to aid in tumor delineation and to rule out additional foci of disease. If additional foci of disease are present, they need to have a negative biopsy to proceed with treatment. 8\. Clinically and radiographically node negative on ultrasound of the axilla or MRI on on initial workup prior to microbubble contrast assessment 9. Estrogen receptor positive or Progesterone receptor positive and Her2neu negative 10. Ability to understand and the willingness to sign a written informed consent. 11\. 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. 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 12\. 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: \- 1. Multi-centric disease 2. Prior Radiation to the involved breast 3. Tumor Size \>3cm 4. 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 5. Prior ipsilateral breast cancer 6. Patients with active lupus or scleroderma 7. History of allergic reactions attributed to compounds of similar chemical or biologic composition to Gadolinium or other agents used in study. 8\. If patient has a positive lymph node at time of microbubble contrast enhanced ultrasound, they will be removed from the study. Only N0 patients to be treated on this study.