Methods:

This was a population-based case-control study of women who underwent radiotherapy for breast cancer in Sweden and Denmark. In Sweden, all women living in Stockholm for whom data were recorded in the Swedish National Cancer Register, who had a diagnosis of breast cancer between 1958 and 2001, were younger than 70 years of age at the time of diagnosis, and had received radiotherapy. In Denmark, all women for whom data were recorded in the register held by the Danish Breast Cancer Cooperative Group were considered for the study if they received the diagnosis of breast cancer between 1977 and 2000, were younger than 75 years at the time of diagnosis, and received radiotherapy. Individual patient information was obtained from hospital records. For each woman, the mean radiation doses to the whole heart and to the left anterior descending coronary artery were estimated from her radiotherapy chart. The primary outcome of interest was major coronary events including myocardial infarction, coronary revascularization, or death from ischemic heart disease.