Partial Breast Irradiation (PBI) for Selected Patients With Early Invasive or Non-Invasive Breast Cancer

* In this study, radiotherapy is given only to a portion of the breast around the tumor bed using external radiation treatments. This approach is called "partial breast irradiation". * Participants will undergo a planning session for the radiation treatments. This planning will be done by CT scan days or weeks before the start of treatment. This planning process is the same as that used to plan conventional radiation therapy treatments. * Radiation therapy will begin 4-12 weeks after the last surgery of the breast in individuals not receiving chemotherapy first. For individuals receiving chemotherapy before radiation therapy, radiation therapy will start 2-6 weeks after ending chemotherapy. * We will be studying three levels of radiation doses to see which is best. The dose the participant receives will depend upon when they are enrolled on the trial. * Participants will receive radiation treatment twice each day for 4 or 5 treatment days, with an overall treatment time of one week.