Breast conservation therapy (BCT) is now widely accepted as a treatment option for most women with Stage I and II invasive breast cancer and most patients with ductal carcinoma in situ (DCIS). Despite superior cosmetic outcome, BCT is more complex and requires a protracted treatment regimen comprised of 6 weeks of daily external beam radiation therapy to the whole breast. The purpose of this study is to determine if an acceptable outcome can be achieved with radiation delivered only to the region of the tumor bed. If this is true, partial breast irradiation may lend itself to much shorter treatment times (one week) and the toxicities to adjacent normal structures (heart, lung, chest wall) will be greatly reduced.