Inclusion Criteria:

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

2. Patients must be >/= 18 years old. (Adenocarcinoma of the breast is not seen in
children)

3. The patient must have stage 0, I, or II breast cancer. If stage II, the tumor size
must be 3 cm or less.

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

5. Surgical treatment of the breast must have been lumpectomy. The margins of the
resected specimen must be histologically free of tumor (DCIS and invasive).
Reexcision of surgical margins is permitted.

6. 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.)

7. Patients with invasive breast cancer are required to have axillary staging which can
include sentinel node biopsy alone (if sentinel node is 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.

8. The target lumpectomy cavity must be clearly delineated and the target lumpectomy
cavity/whole breast reference volume must be less than or equal to 30% based on the
postoperative/pre-enrollment CT scan.

9. Patients are eligible if, based on the postoperative CT scan, PBI is judged to be
technically deliverable.

10. Patients with a history of non-breast malignancies are eligible if they have been
disease-free for 5 or more years prior to enrollment and are deemed by their
physician to be at low risk for recurrence. Patients with the following cancers are
eligible if diagnosed and treated within the past 5 years: carcinoma in situ of the
cervix, carcinoma in situ of the colon, melanoma in situ, and basal cell and squamous
cell carcinoma of the skin.

Exclusion Criteria:

1. Men are not eligible for this study.

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

3. More than 3 histologically positive axillary nodes.

4. Axillary nodes with definite evidence of microscopic or macroscopic extracapsular
extension.

5. Palpable or radiographically suspicious ipsilateral or contralateral axillary,
supraclavicular, infraclavicular, or internal mammary nodes at time of enrollment
unless there is histologic confirmation that these nodes are negative for tumor.

6. Suspicious microcalcifications, or densities (in the ipsilateral or contralateral
breast as documented on mammogram or breast ultrasound) unless biopsied and found to
be benign.

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

8. Proven multicentric carcinoma (invasive cancer or DCIS) in more than one quadrant or
separated by 4 or more centimeters.

9. Paget's disease of the nipple.

10. Synchronous bilateral invasive or non-invasive breast cancer.

11. Surgical margins that cannot be microscopically assessed or are positive at
pathologic evaluation. (If surgical margins are rendered free of disease by
reexcision, the patient is eligible.)

12. Clear delineation of the extent of the target lumpectomy cavity not possible.

13. Treatment plan that includes regional nodal irradiation.

14. Prior radiation to the index breast.

15. Documented diagnosis of collagen vascular disease, specifically dermatomyositis with
a CPK level above normal or with an active skin rash, systemic lupus erythematosis,
or scleroderma.

16. Pregnancy or lactation at enrollment.

17. Women of reproductive potential must agree to use an effective non-hormonal method of
contraception during therapy.