Intensity Modulated Radiotherapy (IMRT) vs. 3D-conformal Accelerated Partial Breast Irradiation (APBI) for Early Stage Breast Cancer After Lumpectomy

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.