The study is being done to determine the tolerance to single fraction intra-operative electron irradiation (IOERT) and external beam photon irradiation (EBRT) following lumpectomy for T1/T2N0M0 breast cancer. IOERT is used in lieu of the 5-8 days of electron beam boost irradiation that is usually given after EBRT. Data from two centers as well as preliminary data from a randomized trial in Europe suggest that IOERT boost is well tolerated and efficacious. In our study, patients will undergo standard lumpectomy and sentinel lymph node sampling or axillary dissection. After the lumpectomy, IOERT of 10 Gy will be delivered in a single fraction to the tumor bed by a mobile electron machine in the operating room. EBRT to the whole breast will be administered once the surgical wound has healed adequately. The primary objective is to determine the feasibility and acute patient tolerance to IOERT and EBRT after lumpectomy. Secondary objectives include assessment of local control and distant control of disease, as well as evaluation of long-term side effects and cosmetic outcome of the treatment.