Inclusion Criteria:

Women who satisfy all of the following conditions are the only patients who will be
eligible for this study:

1. Patient must have invasive ductal, medullary, papillary, colloid (mucinous), or
tubular histologies.

2. AJCC Stage I or IIA (T1N0 or T2N0) histologically confirmed invasive carcinoma of the
breast with a primary lesion (less than/equal to 3 cm)by MRI or ultrasound

3. Clinically negative axillary lymph nodes. Standard routine imaging assessment of the
axilla is performed by either ultrasound or MRI. If lymph nodes are suspicious, a
biopsy is required. An involved axilla is purposely excluded.

4. Unifocal breast cancer (single focus which can be encompassed by one lumpectomy).

5. Patients must have an estrogen receptor (ER), progesterone receptor (PR), and human
epidermal growth receptor 2 (H2N) analyses performed on the primary tumor prior to
enrollment.

6. Based on pre-treatment planning CT scan, the ratio of the PTV to the reference breast
volume should not exceed 25%.

7. Patients must be greater than/equal to 18 years of age.

8. Pretreatment evaluations required for eligibility include: bilateral mammogram,
histologic confirmation of malignancy, and physical exam.

9. Signed study-specific informed consent form prior to study entry.

10. Women of childbearing potential must use an effective contraceptive method such as
condom/diaphragm and spermicidal foam, intrauterine device (IUD), or prescription
birth control pills.

Exclusion Criteria:

Men are not eligible for this study. Women with one or more of the following conditions
also are ineligible for this study:

1. Evidence of suspicious microcalcifications which are separate from the known lesion
unless pathologically confirmed to be benign.

2. Patient with distant metastases.

3. Patients with invasive lobular or extensive in-situ lobular carcinoma or pure ductal
carcinoma in-situ or non-epithelial breast malignancies such as sarcoma or lymphoma.

4. Patients with proven multicentric carcinoma (tumors in different quadrants of the
breast or tumor separated by at least 4 cm) or with other clinically or
radiographically suspicious areas in the ipsilateral breast unless confirmed to be
negative for malignancy by biopsy.

5. Patient whose tumor is not visible on radiation treatment planning CT scan

6. Palpable or radiographically suspicious ipsilateral axillary, supraclavicular,
infraclavicular or internal mammary nodes, unless there is histologic confirmation
that these nodes are negative for tumor.

7. Prior hormonal or non-hormonal therapy or radiation therapy for the current breast
cancer.

8. Patients with Paget's disease of the nipple.

9. Patients with skin involvement, regardless of tumor size.

10. Patients with a breast technically unsatisfactory for radiation therapy.

11. Patients with collagenous diseases, specifically systemic lupus erythematosis,
scleroderma, or dermatomyositis.

12. Patients with co-existing medical conditions with life expectancy less than 2 years.

13. Patients with psychiatric or addictive disorders that would preclude obtaining
informed consent.

14. Patients who are pregnant or lactating due to potential exposure of the fetus to RT
and unknown effects of RT to lactating females.