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 or invasive breast carcinoma. For
patients with invasive breast cancer, an axillary staging procedure must be
performed. Either:

1. sentinel node biopsy (SNB) alone if sentinel node(s) is/are negative; or

2. axillary dissection (minimum of six axillary nodes removed); and

3. the axillary node(s) must be pathologically negative.

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

- Estrogen receptor positive tumor

Exclusion Criteria:

- Age < 50 at diagnosis (regardless of histology)

- Pregnant or breast-feeding (if appropriate, patient must use birth control during the
study)

- Active collagen-vascular disease

- Paget's disease of the breast

- History of DCIS or invasive breast cancer

- Prior breast or thoracic RT 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 or
close

- Positive axillary node(s)

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

- Estrogen receptor negative tumor