Conservative surgery followed by radiotherapy is the current treatment of choice for primary breast cancer: indeed, this protocol ensures local control, relatively good cosmetic results and NED survival values similar to those of more invasive surgery. Radiotherapy requires the optimization of the irradiation technique to minimize the dose to the organs at risk. To this purpose, 30 patients submitted to quadrantectomy for breast carcinoma and then to radiotherapy on the residual breast were examined, January through December, 1994, at the Radiotherapy Service of the Ivrea Hospital, to investigate if CT can help optimize treatment planning, sparing as much of the pulmonary tissue underlying the residual breast as possible. Our series of patients was then compared with a literature series whose treatment had been planned only on the mapping of body outline. Some interesting considerations follow from our results: 1) With CT, larger fields can be used than those used with the body outline, so that the planned target volume can be more closely approached; 2) Larger fields can be used because the critical organ included in the irradiation field is more correctly and precisely defined; 3) The comparison with the literature shows that in the past the fields were larger, because the target volume and the critical organ were more difficult to define. The use of larger fields means a higher dose to the lung and thus maybe a higher risk of radiation pneumonia.