Inclusion Criteria:

- Life expectancy of at least two years.

- Stage 0, I, or II breast cancer. If stage II, the tumor size must be 3 cm or less.

- On histological examination, the tumor must be DCIS or invasive adenocarcinoma of the
breast.

- Surgical treatment of the breast must have been lumpectomy. The margins of the
resected specimen must be histologically free of tumor (>2mm, DCIS and invasive).
Re-excision of surgical margins is permitted.

- Gross disease must be unifocal with pathologic (invasive and/or DCIS) tumor size 3 cm
or less. (Patients with microscopic multifocality are eligible as long as total
pathologic tumor size is 3 cm or less.)

- Patients with invasive breast cancer are required to have axillary staging which can
include sentinel node biopsy alone (if negative), sentinel node biopsy followed by
axillary dissection or sampling with a minimum total of 6 axillary nodes (if sentinel
node is positive), or axillary dissection alone (with a minimum of 6 axillary nodes).
(Axillary staging is NOT required for patients with DCIS.)

- The patient must have the MammoSite catheter placed within 4 weeks or 28 days of the
final surgery for their breast cancer (lumpectomy, re-excision of margins, or
axillary staging procedure). Placement of a spacer for the MammoSite catheter is
permitted at their final surgery.

- Patients with a history of non-ipsilateral breast malignancies are eligible if they
have been disease-free for 2 or more years prior to randomization. Patients with the
following cancers are eligible even if diagnosed and treated within the past 2 years:
carcinoma in situ of the cervix, colon, melanoma in situ, and basal cell and squamous
cell carcinoma of the skin. Patients with a prior diagnosis of ipsilateral breast
cancer are ineligible.

- Must have 3 or fewer histologically positive axillary nodes with no extracapsular
extension.

- Chemotherapy is permitted if planned for ≥2 weeks after removal of Mammosite
catheter.

- Patient must be ineligible or have refused enrollment on the randomized trial RTOG
0413.

Exclusion Criteria:

- T2 (>3.0 cm), T3, stage III or IV breast cancer.

- More then 3 histologically positive axillary nodes.

- Axillary nodes with evidence of extracapsular extension.

- Palpable or radiographically suspicious ipsilateral or contralateral axillary,
supraclavicular, infraclavicular or internal mammary nodes, unless biopsy proven to
be negative for tumor.

- Suspicious microcalcifications, densities or palpable abnormalities in either breast
unless biopsy proven to be benign.

- Non-epithelial breast malignancies such as sarcoma or lymphoma.

- Proven multicentric carcinoma in more than one quadrant or separated by more than 3
centimeters.

- Paget's disease of the nipple.

- History of invasive breast cancer or DCIS in the same breast.

- Surgical margins that cannot be microscopically assessed or are less then 2 mm.

- Collagen vascular disease, specifically dermatomyositis with a CPK level above normal
or with an active skin rash, systemic lupus erythematosis or scleroderma.

- Pregnancy or lactation at the time of proposed radiation. Women of reproductive
potential must agree to use an effective non-hormonal method of contraception during
therapy.

- Psychiatric or addictive disorders or other conditions that, in the opinion of the
investigator, would preclude the patient from meeting the study requirements.

- Patients with coexisting medical conditions in whom life expectancy is < 2 years.

- Patients with skin involvement, regardless of tumor size.

- Patients for whom treatment with the MammoSite catheter is not feasible, such as
those with too little breast tissue between the skin and the catheter (<5mm).