A Phase I Dose Escalation Study of Single Fraction Pre-operative Partial Breast (S-PBI) for Early Stage Breast Cancer

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
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
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.If patient cannot have MRI, contrast enhanced digital mammography (CEDM) is allowed in place of MRI.
8. Clinically and radiographically node negative on ultrasound of the axilla or MRI
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

Exclusion Criteria:

1. Multi-centric disease
2. Prior RT to the involved breast
3. Tumor size \>3cm
4. Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that, in the opinion of the investigator, would limit compliance with study requirements
5. 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
6. Patients unable to have an MRI or contrast enhanced digital mammography (CEDM)
7. Prior ipsilateral breast cancer
8. Tumor less than 5mm from the skin surface on clinical exam and/or radiographic imaging
9. Patients with active Lupus or scleroderma