In the setting of radiotherapy as part of breast-conservation therapy for patients with early stage breast cancer, the novel planning and delivery method of intensity modulated radiotherapy is an effective and safe alternative to the commonly-used standard 3D-conformal external beam radiotherapy, spares more normal breast and lung tissue, and may lead to improved clinical outcomes.

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.