Up to 200 patients with biopsy proven early stage breast cancer selecting Wide Local Excision (WLE) will undergo mammography, ultrasound (of affected breast and ipsilateral axilla) and contrast enhanced-magnetic resonance imaging (CE-MRI) of the affected breast to evaluate the extent of disease.

Patients will be deemed eligible for immediate IORT if the invasive cancer lesion is estimated to measure ≤ 3.0 centimeters maximum dimension by mammogram, ultrasound, and CE-MRI and is thought to be resectable with WLE and clear surgical margins, with clinically staged N0M0 disease, minimum age of 48, and histology of invasive ductal or lobular carcinoma of the breast. The lesion size determined by imaging will be compared with lesion size and surgical margin status obtained from the surgical pathology specimen to evaluate the ability of mammography combined with ultrasound and CE-MRI to identify suitable candidates for immediate IORT.

Inclusion Criteria:

1. Informed Consent. Each eligible patient must willingly give written consent after being informed of the procedures to be followed, the experimental nature of the therapy, alternatives, potential benefits, side effects, risks, and discomforts.
2. Histology. Patients must have biopsy-proven invasive ductal or lobular carcinoma of the breast.
3. Female, age ≥ 48 years.
4. Clinically and/or histologically negative axillary lymph nodes.
5. No imaging or clinical findings that indicate metastatic disease.
6. Procedure deemed appropriate based upon factors such as breast size by the treating surgeon and radiation oncologist.

Exclusion Criteria:

1. Male sex
2. Age < 48
3. Non-epithelial breast malignancies such as sarcoma, lymphoma, or metastatic breast cancer
4. Pregnancy or lactation
5. Serious psychiatric or addictive disorders
6. More than one obvious cancer in the same breast as diagnosed by clinical examination, mammography, ultrasonography, and/or CE-MRI.
7. Bilateral breast cancer at the time of diagnosis, unless agreed upon by the surgeon and radiation oncologist.
8. Ipsilateral breast with a previous cancer and/or irradiation.
9. Extensive Intraductal Component (DCIS ≥ 25% of the tumor).
10. Patients presenting with gross nodal disease considered to be clinically malignant by scanning or proven cytologically or by sentinel node biopsy will be cancelled.
11. Previous history of malignant disease does not preclude entry if the expectation of relapse-free survival at 10 years is 90% or greater.
12. Lymphovascular invasion on needle biopsy.
13. Patients undergoing primary medical treatment (hormones or chemotherapy) as initial treatment with intent to reduce tumor size.