A population-based case-cohort study of the risk of myocardial infarction following radiation therapy for breast cancer

Objective — To describe the risk of acute myocardial infarction (AMI) after radiation therapy (RT) for breast cancer (BrCa) in an exposed population.

Methods — We identified and validated cases of AMI (vAMI), by electrocardiographic or enzyme criteria, among all 6680 women who received post-operative RT following lumpectomy or mastectomy, within 12 months following diagnosis of BrCa between 1982 and 1988 in Ontario, Canada. We identified women without vAMI whose death certification was ascribed to AMI (dAMI). We abstracted risk factors and treatment exposures for a random sample of women from the 6680, and for all with vAMI or dAMI. The hazards of vAMI and of dAMI were estimated using multivariate Cox proportional hazards models, corrected for study design.

Results — We validated 121 cases of vAMI and identified 92 cases of dAMI. The risk of vAMI associated with RT to the left breast HR = 1.96 (1.09,3.54) among women at age ⩾ 60 at time of RT, adjusted for history of smoking and prior MI. The adjusted HR dAMI = 1.90 (1.08,3.35) for exposure to anterior internal mammary node (IMC) RT. Among women who received anterior left breast boost RT, increasing area of the boost is associated with adjusted HR vAMI = 1.02 (1.00,1.03)/cm2, and adjusted HR dAMI = 1.02 (1.01,1.03)/cm2.

Conclusion — The risks of vAMI and dAMI following RT for BrCa are related to anatomic sites of RT (left breast, area of anterior left breast boost field, and anterior IMC field).