Inclusion Criteria:

- Able and willing to sign informed consent

- Age 50 or older at diagnosis

- Life expectancy greater than 10 years (excluding diagnosis of breast cancer)

- Surgical treatment of the breast must have been lumpectomy. The margins of the
resected specimen must be histologically free of tumor (negative surgical margins per
NSABP criteria)

- On histologic examination, the tumor must be DCIS and/or invasive breast carcinoma

- For patients with invasive breast cancer, an axillary staging procedure must be
performed [either sentinel node biopsy or axillary dissection (with a minimum of 6
axillary nodes removed), and the axillary node(s) must be pathologically negative]

- The T stage must be Tis, T1, or T2. If T2, the tumor must be less than or equal to
3.0 cm in maximum diameter

- Estrogen receptor positive tumor

Exclusion Criteria:

- Age < 50 at diagnosis (regardless of histology)

- Pregnant or breast-feeding

- Active collagen vascular disease

- Paget's disease of the breast

- Prior history of DCIS or invasive breast cancer

- Prior breast or thoracic radiation therapy for any condition

- Multicentric carcinoma (DCIS or invasive)

- Synchronous bilateral invasive or non-invasive breast cancer

- Surgical margins that cannot be microscopically assessed or that are positive

- Positive axillary node(s)

- T stage of T2 with the tumor > 3 cm in maximum diameter or a T stage of T3 or T4

- Estrogen receptor negative tumor