Inclusion Criteria:

1. A confirmed histological diagnosis of invasive breast carcinoma or DCIS

2. Age greater or equal to 50 years old

3. Life expectancy > 6 months

4. Treated by breast conserving surgery with axillary node dissection or sentinel
lymph node biopsy

5. Pathologic tumor size less than or equal to 2 cm

6. Invasive ductal, mucinous, tubular or colloid histology

7. Estrogen receptor positive for invasive carcinoma.

8. Unifocal/unicentric disease

9. Negative surgical margins greater than or equal to 2 mm

10. Pathologic lymph node negative

11. No evidence of lymphovascular invasion

12. ECOG performance status of 0 or 1 (Appendix 1)

13. Informed consent signed.

Exclusion Criteria:

1. Known BRCA 1/2 Mutation; (BRCA 1 and 2 testing is not required)

2. Autoimmune disorder

3. Pregnancy

4. Breast implants

5. Psychiatric or addictive disorder that would preclude attending follow-up

6. Neoadjuvant chemotherapy (adjuvant chemotherapy is permitted)

7. Suspicious remaining microcalcification on post-surgery mammogram (unless biopsy
proven benign)

8. Lobular features on histology (pure or mixed) or sarcoma histology

9. Node positive on axillary dissection or in the sentinel lymph node biopsy;

10. Extensive in situ carcinoma (EIC)

11. Multicentric or multifocal disease

12. Paget's disease of the nipple

13. Distant metastases

14. Lumpectomy cavity not well visualized on AccuBoost imaging

15. Lumpectomy cavity with 1cm margin (PTV) not adequately encompassed by any applicator
(PTV > 6cm)

16. Breast separation with compression > 7cm.

17. Overlap of skin between orthogonal treatment axes.