RAD 1802: Pilot Trial of Five Fraction Stereotactic Body Radiotherapy for Early Stage Breast Cancer Patients Eligible for Post-Operative Accelerated Partial Breast Irradiation (APBI)

Inclusion Criteria: * Pathologically proven invasive mammary carcinoma, Invasive ductal carcinoma (IDC) or ductal carcinoma in situ (DCIS) of the breast. Medullary, papillary, mucinous (colloid) and tubular histologies are allowed. * Age\>50. * Maximum pathologic tumor size \<2.0cm if invasive carcinoma or \< 2.5cm if pure DCIS. * Estrogen receptor (ER) positive (\>10%). * Must be eligible for breast conservation therapy and receive a lumpectomy with pathologic margins of at least 2mm. * Must be clinically node negative by physical examination. Sentinel node dissection is not required, but if undertaken, the patient must be pathologically node negative. * Zubrod Performance Status 0-2. Exclusion Criteria: * Multifocal or multicentric cancer. * Reception of neoadjuvant chemotherapy. * Pure invasive lobular histology. * Inability to clearly delineate lumpectomy cavity on post lumpectomy planning scan. * Measured maximum PTV of \>124cc. * Lumpectomy cavity within 5mm of body contour.