 Patients will undergo a pre-treatment planning free breathing CT with 5mm cuts in the treatment position, on a breast board, with both arms extended above their head that will be used to plan the traditional two field tangent plan. An Active Breathing Control ABC device will be used to minimize target motion due to breathing during a second CT scan. The lumpectomy cavity will be identified and delineated, and margins of approximately 1.5 cm in three dimensions will be used to allow for subclinical tumor extension and daily set-up error. The organs at risk heart, lungs, contralateral breast will be contoured on both CT scans. An inverse planning system and optimization tools will be employed in order to achieve the best IMRT plan to minimize the dose to the heart, lungs and contralateral breast, that will be compared to the two field plan. If the IMRT plan is shown to be the optimal one, in terms of doses to organs at risk, then the patient will be enrolled in the protocol and will proceed with the treatment. Treatment will start approximately two weeks after the planning CT is obtained. All patients on protocol will be treated with accelerated radiotherapy, 3.85 Gy per fraction, bid, for 5 consecutive days for a total dose of 38.5 Gy. 