Inclusion Criteria:

- The patient must consent to be in the study and must have a signed an approved
consent form conforming with institutional guidelines.

- Patient must be > 50 years old.

- The patient should have a life expectancy of at least two years with a karnofsky
performance status > 70.

- The patient must have stage 0 or I breast cancer.

- 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 2 cm
or less. (Patients with microscopic multifocality are eligible as long as total
pathologic tumor size is 2 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 or axillary
dissection alone (with a minimum of 6 axillary nodes). Axillary staging is NOT
required for patients with DCIS.

- The patient must have simulation within 8 weeks/56 days of the final surgery for
their breast cancer (lumpectomy, re-excision of margins, or axillary staging
procedure).

- Patients with a history of non-ipsilateral breast malignancies are eligible if they
have been disease-free for 2 or more years prior to enrollment. 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.

- Chemotherapy is permitted if planned for ≥ 2 weeks after radiation.

- Urine pregnancy test must be performed and be negative on all women younger than 60
who have not had a tubal ligation, oophorectomy, or hysterectomy.

- Separate incisions for the lumpectomy and sentinel node biopsy should be present. Use
of only one incision will typically result in a contiguous cavity with the tumor bed
and the sentinel node sampling, and inability of the radiation oncologist to
delineate the tumor bed from the sentinel node bed.

- The patient must have a cavity which is able to be targeted with external beam APBI,
either through surgical clip placement, or CVS 3 or higher. The cavity to whole
breast ratio must be 30% or less.

Exclusion Criteria:

- Men are not eligible for this study as men are not breast conservation candidates.

- T0, T2 (> 2.0 cm), T3, node positive, stage III or IV breast cancer.

- Any positive axillary nodes.

- 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 2
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
treating physician, 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.