CARDIAC DOSES IN POSTOPERATIVE BREAST IRRADIATION

Data were collected on radiation doses given to the heart and coronary arteries during primary breast irradiation in order to analyze factors which might be important in the aetiology of subsequent cardiac-related disease. Twenty eight patients with breast cancer were studied. Fourteen patients treated from 1957 to 1984 were studied retrospectively (group 1), and 14 treated from 1988 to 1989 were studied prospectively (group 2). All patients had stage I or II disease at presentation, and were under 70 years of age. None had chemotherapy as a primary form of treatment. Patients were given a computed tomography scan of the chest, and three-dimensional reconstruction was made of the heart, lung and body contour. Original dose distributions were super-imposed on these outlines, and doses to the total cardiac volume and three main coronary arteries were estimated using an alpha/beta ratio of 4 Gy. Nine out of 14 patients in group 1 had a mastectomy followed mainly by orthovoltage radiation with similar techniques used up until 1984. Thirteen out of 14 patients in group 2 had conservative surgery followed by a modern two- or four-field megavoltage technique. We found that for patients with left-sided tumours (n = 20), the heart volume irradiated to a minimum extrapolated target dose of 5 Gy is significantly decreased for patients treated with a modern technique (group 2) when compared with those treated with earlier techniques (group 1). The doses to the left circumflex (LCx) and right coronary arteries (RCA) are also significantly reduced, but the dose to part of the left anterior descending artery (LAD) remains unchanged, and receives a substantial proportion of the target dose in both groups. The megavoltage techniques have been shown to reduce the total heart dose and to spare the LCx and RCA, but new radiation techniques would be needed to reduce the dose to the LAD. It is not yet known whether radiation-induced atherosclerosis of this vessel contributes to cardiac morbidity.