Inclusion Criteria:

- -Histologically confirmed diagnosis of invasive or non-invasive breast cancer

- Invasive ductal, medullary, papillary, colloid (mucinous) or tubular histologies

- AJCC T1 or T2; N0 or N1mic; Stage IA-IIA breast cancer

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

- Estrogen and/or progesterone receptor positive

- No evidence of distant metastatic disease as documented by history and physical
examination (radiographic staging only to be performed as indicated by symptoms or
physical findings).

- Patients must have an ECOG Performance Status of 0, 1, or 2.

- Age greater than or equal to 50

- Patients must be able to provide informed consent

- Patients must have undergone breast-conserving surgery

- All tumors (invasive and non-invasive disease) must be excised with a minimum margin
width of greater than or equal to 2 mm. Re-excision of surgical margins is permitted.
Focally close (2mm) or positive (tumor cells at the inked edge of the specimen)
margins determined to be at an anatomic boundary of resection by surgeon, such as
posterior fascia for posterior margins or skin for anterior margins, are also
acceptable.

- Patients must be node-negative (N0) or have only microscopic disease (less than or
equal to 2mm) in the nodes (N1mi). Patients are required to have axillary staging.

Options for axillary staging include:

1. Negative sentinel lymph node biopsy (SLNB)

2. Level I-II axillary lymph node dissection (ALND) (6 or more nodes removed)

3. Positive SLNB followed by completion ALND (6 or more nodes removed)

- Patients presenting with abnormal microcalcifications on a screening mammogram
must have radiographically confirmed excision of the suspicious
microcalcifications, either by specimen radiograph or post-biopsy mammograms.

- The patient must be enrolled on the study within 42 days following the last
surgery for breast cancer (lumpectomy, re-excision of margins, or axillary
staging procedure).

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

- Patients must have bilateral mammogram and/or breast MRI within 3 months of
diagnosis of their breast cancer.

- CBC/differential obtained within 3 months prior to registration on study, with
adequate bone marrow function defined as follows: Absolute neutrophil count
(ANC) 1,800 cells/mm3; Platelets 75,000 cells/mm3; Hemoglobin 8.0 g/dl (Note:
The use of transfusion or other intervention to achieve Hgb 8.0 g/dl is
acceptable).

- Patients with synchronous bilateral breast cancers who will be treated with
radiotherapy to each breast are eligible, provided such treatment can be
performed in a manner that avoids overlap between treatment fields. Both sides
may be treated with APBI if the pathologic eligibility criteria are met for both
tumors, or only one side may be treated with APBI if the criteria are met for
only one tumor.

- Patients with a history of prior breast cancer in the opposite breast are
eligible as long as treatment can be performed without overlapping any prior RT
fields.

- Patients with a history of prior breast cancer in the ipsilateral breast treated
with lumpectomy alone (no RT) are eligible as long as the other entry criteria
for this study are met.

Patients with a history of non-breast malignancies are eligible as long as they have not
received prior radiotherapy to the thoracic region, and have a greater than 2 year
interval without evidence of recurrence.

- Women of childbearing potential must be non-pregnant and non-lactating and willing to
exercise an effective form of birth control during radiation therapy (e.g. oral
contraceptive, IUD, condoms or other barrier methods etc.). Hysterectomy or menopause
must be clinically documented.

- Patient must provide study-specific informed consent prior to study entry

Exclusion Criteria:

- -Male breast cancer

- T2 (3cm), T3, T4, Node positive (other than N1mi), or M1 disease

- Pure DCIS

- Lobular or mixed ductal and lobular histology.

- Multifocal primary tumor.

- HER2-neu positive.

- Breast cancers with an extensive in-situ ductal component (defined as 25% of the
tumor volume and DCIS present within and in the surrounding normal breast tissue of
the invasive tumor).

Clear delineation of the extent of the lumpectomy cavity is not possible.

- Prior or simultaneous malignancies within the past two years (other than carcinoma in
situ of the cervix, CIS of the colon, melanoma in situ, thyroid cancer, and basal
cell or squamous cell carcinoma of the skin).

- Any non-axillary sentinel node(s) positive. (Note that intramammary nodes are staged
as axillary nodes).

- Patients who have had a positive SLNB but decline completion ALND are not eligible.

- Patients treated with neoadjuvant chemotherapy are not eligible.

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

- Suspicious microcalcifications, densities, or palpable abnormalities (in the
inpsilateral or contralateral breast) unless these were biopsied and found to be
benign.

- Proven multicentric carcinoma (invasive cancer or DCIS) in more than one quadrant or
two or more breast cancers not resectable through a single lumpectomy incision.

- Pagets disease of the nipple. Surgical margins that cannot be microscopically
assessed or are positive at pathological evaluation. A focally positive margin
determined to be at an anatomic boundary of resection by the surgeon, such as
posterior fascia for posterior margins and skin for anterior margins, is also
acceptable. If surgical margins are rendered free of disease by re-excision, the
patient is eligible.

- Breast implants. (Patients who have implants removed are eligible).

- Prior ipsilateral breast or thoracic radiation for any condition.

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

- Pregnant women, women planning to become pregnant and women that are nursing.

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

- Actively being treated on any other therapeutic research study.