The gross tumor volume (GTV) will be defined by lumpectomy cavity contoured on each CT slice. The clinical target volume (CTV) will consist of GTV uniformly expanded in three dimensions by 1cm; however the volume will be constrained to lie 5mm within external contour and up against the pectoralis major muscles. The planning target volume (PTV) will be calculated from the CTV using uniform three dimensional expansion of 0.5 cm. The ipsilateral whole breast will be defined to lie within the radioopaque markers and as deep as the anterior chest wall muscles. The cranial extent of heart will include the infundibulum of right ventricle, the right atrium and right auricle but exclude the pulmonary trunk, ascending aorta and superior vena cava. The lowest external contour of heart will be the caudal border of mediastinum. The pericardium should be excluded from the heart volume. Both lungs will be contoured. The contralateral breast will be contoured as the breast parenchyma is visible on CT images.