Inclusion Criteria:

- Pathologically confirmed stage 0, I or II bilateral or unilateral breast cancer

- Any invasive adenocarcinoma or intraductal carcinoma

- Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2

- Surgical treatment with lumpectomy (partial mastectomy)

- Successful placement of intraparenchymal metallic markers at last breast surgery

- Pathologic tumor size < 5.0 cm (Microscopic multifocality is allowed if total
pathologic tumor size is within 5 cm)

- Axillary sampling (sentinel node or axillary dissection) performed for axillary nodal
staging for all invasive cancers. No axillary sampling is required for ductal
carcinoma in situ)

- Negative margins of excision.

- Radiation treatment to begin within 8 weeks after last surgery or last dose of
chemotherapy

- Negative post-biopsy mammogram if presented with mammographically detected
microcalcifications to ensure removal of suspicious calcifications

- History of prior cancers is allowed if patient is without evidence of disease at the
time of study entry

- Ability to understand and the willingness to sign written informed consent document

Exclusion Criteria:

- Pregnant or breast feeding at time of study entry. Note: Radiation therapy is
teratogenic. Women of child bearing potential must agree to use adequate
contraception (abstinence, hormonal or barrier method of birth control) prior to and
during study participation. Should a woman become pregnant, she should inform the
treating physicians immediately.

- Prior in-field irradiation

- Stage III or IV breast cancer

- Inability to place intraparenchymal metallic markers due to excessive bleeding or
other intraoperative complication so that the surgeon deems it inadvisable to place
the marker

- Pathologic tumor size >/= 5 cm

- Positive or unassessed margins of surgical resection

- Diffuse calcifications on mammogram pre- or post-operatively

- Positive or suspicious post-lumpectomy mammogram or breast magnetic resonance imaging
(MRI)

- Multicentric carcinoma in more than one quadrant of the breast

- Non-epithelial breast malignancy; Paget's disease of the nipple

- Personal history of collagen vascular disease clinically judged to be a
contraindication to radiation therapy

- Recurrent disease or prior history of ipsilateral breast cancer

- Psychiatric or addictive disorders that impair patient's voluntary ability to
participate in informed consent or protocol procedures