The objective of this study was to determine whether diagnostic interpretation of contrast-enhanced computed tomography (CT) images for radiation planning in breast irradiation detects a considerable number of unexpected abnormalities.One hundred thirty-one patients underwent treatment-planning CT scans for breast or chest-wall irradiation. A diagnostic radiologist prospectively reviewed each scan and determined the incidence of previously unknown findings, the impacts of such findings on treatment, and the need for additional radiological studies based on the CT interpretation.Eighty-six scans were prospectively classified as negative findings, 27 scans were classified as incidental benign findings, 26 scans were classified as medically important findings of which only one case was metastatic.The incidence of clinically important coexistent diseases in CT scans for radiation planning in breast irradiation is very low, although the CT scans were performed with intravenous contrast administration insofar as the preoperative evaluation was appropriate.