Mortality from myocardial infarction following postlumpectomy radiotherapyfor breast cancer: A population-based study in Ontario, Canada

Purpose: To compare the risk of mortality from myocardial infarction (MI) a fter left-sided postlumpectomy radiotherapy (RT) to the risk after right-si ded postlumpectomy RT. Methods: We conducted a population-based cohort study of cases of invasive female breast cancer in Ontario, diagnosed between January 1, 1982 and Dece mber 31, 1987 (n = 25,570). Records of the Ontario Cancer Registry (OCR) we re linked to hospital procedure and discharge abstracts and to RT records f rom Ontario cancer centers. A case was labelled as lumpectomy if this was t he maximum breast surgery within 4 months of diagnosis. Postlumpectomy RT o ccurred up to 1 year postdiagnosis. Laterality was assigned from the latera lity descriptor of the RT records. A case was labelled as having had a fata l MI if ICD code 410 (myocardial infarction) was recorded as the cause of d eath in the OCR. We used logistic regression to compare the likelihood of u tilization of: 1. Dose per fraction > 2.00 Gy; 2. cobalt vs. linac; and 3. boost RT. We used life table analysis and the log rank test comparing the t ime to fatal MI from diagnosis of breast cancer between women who received left-sided postlumpectomy RT and women who received right-sided. We used Co x proportional hazards models to study the relative risk for left-sided cas es overall, and stratified by age, RT characteristics, and among conditiona l survival cohorts. Results: Postlumpectomy RT was received by 1,555 left-sided and 1,451 right -sided cases. With follow-up to December 31, 1995, 2% of women with left-si ded RT had a fatal MI compared to 1% of women with right-sided RT. Comparis on of the time to failure between women who had left-sided RT and women who had right-sided RT showed the left-sided RT group to be associated with a higher risk of fatal MI (p = 0.02). Adjusting for age at diagnosis, the rel ative risk for fatal MI with left-sided postlumpectomy RT was 2.10 (1.11, 3 .95). Conclusion: Among women who received postlumpectomy RT for breast cancer in Ontario between 1982-1987, left-sided postlumpectomy RT was associated wit h a higher risk of fatal MI compared to right-sided. (C) 1999 Elsevier Scie nce Inc.