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