Breast cancer remains the commonest cancer in women with approximately 1 million new cases diagnosed annually. However, survival rates have increased year-on-year with 5 year disease free survival as high as 90% in women in whom the tumour is diagnosed early. Large numbers of cancer survivors are therefore present in the population – an estimated three million women in North America alone. While anthracyclines and therapies targeting the HER2 receptor are known to cause cardiac muscle disease, the long term cardiac effects of radiotherapy – a common component of many treatment strategies – is less well explored.
In this population-based case-control study Darby et al. utilised comprehensive registry data from Sweden and Denmark following 2168 women who underwent radiotherapy for breast cancer between 1958 and 2001. The primary end-point was a major coronary event (i.e., myocardial infarction, coronary revascularization, or death from ischemic heart disease). Individual patient information was obtained from hospital records and the mean radiation doses to the whole heart and to the left anterior descending coronary artery were estimated from radiotherapy charts.
963 women had major coronary events during follow up. Mean dose to the whole heart was 4.9 Gy (range, 0.03 to 27.72). Rates of major coronary events increased linearly with mean dose by 7.4% per gray (95% CI, 2.9 to 14.5; P<0.001), with no apparent threshold. The increase started within the first 5 years after radiotherapy and continued into the third decade after radiotherapy. The proportional increase in the rate of major coronary events per gray was similar in women with and women without cardiac risk factors at the time of radiotherapy.
Exposure of the heart to ionizing radiation during radiotherapy for breast cancer increases the subsequent rate of ischemic heart disease with the risk being proportional to the mean dose to the heart.
- Darby SC, Ewertz M, McGale P et al. Risk of ischemic heart disease in women after radiotherapy for breast cancer. N Engl J Med. 2013 Mar 14;368(11):987-98.