Inclusion Criteria:

- Histologically confirmed disease (AJC Classification): Tis, T1, T2 (≤ 3.0 cm), N0,
M0.

- Microscopic multifocal disease is only allowed when the entire span of identified
disease measures 3.0 cm or less.

- Negative surgical margins ( ≥ 0.2 cm) after final surgery.

- Subjects with infiltrating lobular histologies or high nuclear grade DCIS will be
required to have breast MRI scanning as part of the initial staging to verify
localized disease.

- Subjects with DCIS will be included in the study only if they had an MRI prior to
lumpectomy.

- Findings on MRI scanning revealing relevant suspicion of disease outside of planned
lumpectomy volume should be further evaluated by ultrasound and, if necessary biopsy,
to exclude multicentric/multifocal disease.

- Subjects with malignant calcifications on mammography will be required to have repeat
mammography after surgery to ensure removal of all malignant calcifications.

- Willing to complete additional screening requirements and meet eligibility criteria
as defined in protocol Sec. 4.4.

- Successful placement of fiducial markers for IGRT requiring nonmigrating fiducials.

- PTV to ipsilateral breast ratio (IBR) ≤ 25 %.

- Radiotherapy anticipated to begin within 10 weeks of lumpectomy or re-excision of
margins.

Exclusion Criteria:

- Pregnancy or breast-feeding.

- Have collagen-vascular disease.

- Inadequate surgical margins ( < 0.2 cm) after final surgery.

- Subjects with persistent malignant/suspicious micro-calcifications.

- Gross multifocal disease and microscopic disease greater than 3.0 cm.