1. Female subject with a pathologically-confirmed diagnosis of invasive ductal carcinoma.
2. Tumor size \< 2cm.
3. Negative axillary node involvement, determined by either
1. Sentinel node biopsy,
2. Axillary node dissection removing \> 6 nodes
4. No evidence of metastatic disease.
5. Treated by BCS with microscopically clear resection margins (\> 2mm to ink) for invasive and non-invasive disease or no residual disease on re-excision.
You may not be if
1. Age \< 60 years.
2. Subjects who will receive adjuvant chemotherapy.
3. A known deleterious mutation in BRCA-1 and/or BRCA-2.
4. Extensive DCIS, mammographically or pathologically.
5. Tumor histology of pure lobular carcinoma (invasive or in-situ).
6. Adenoid cystic and non-epithelial malignancies (e.g. sarcoma or lymphoma).
7. Lymphovascular invasion (LVI) present.
8. Bilateral invasive malignancy of the breast (synchronous or metachronous).
9. More than one ipsilateral breast primary tumor.
10. Previous irradiation to the ipsilateral breast.
11. Presence of an ipsilateral breast implant.
12. Serious non-malignant disease that precludes radiation treatment or conscious sedation.
13. Unable for any reason to undergo PBSI within 14 weeks of the last breast surgery.
14. Concomitant malignancies, except non-melanoma skin cancer and carcinoma in situ of any site.
