Rates of myocardial infarction and coronary artery disease and risk factors in patients treated with radiation therapy for early-stage breast cancer.

BACKGROUND: Radiation therapy (RT), a critical component of breast-conserving therapy for breast cancer, has been associated with coronary artery disease (CAD) in numerous older studies, but the risk may be lower with modern techniques.
METHODS: Observed rates of cardiac events in 828 patients treated with breast-conserving surgery and RT at the University of Michigan were compared with expected rates. Relations between potential risk factors and actuarial rates of first CAD event were analyzed.
RESULTS: Observed risks of cardiac events were lower than expected. The standardized incidence ratio (SIR) of myocardial infarction (MI) was 0.44 (95% confidence interval [CI]: 0.21-0.70). The SIR of MI or CAD requiring intervention was 0.50 (95% CI: 0.27-0.68). With a median follow-up of 6.8 years, 12 (1.4%) patients had at least 1 MI on follow-up and 20 (2.4%) had at least 1 MI or CAD requiring intervention. Median age at first cardiac event was 75.9 years (range, 43.1-91.5). Median interval from RT to occurrence of the first cardiac event was 3.7 years (range, 13 days to 15.4 years). The 10-year cumulative incidence of MI was 1.2% and cumulative incidence of MI or CAD requiring intervention was 2.7%. On multivariate analysis, age, diabetes mellitus, active smoking, and laterality of RT were significant predictors of MI. Age and active smoking were significant predictors of MI or CAD requiring intervention.
CONCLUSIONS: Patients in this series had lower risk of ischemic cardiac events than expected. Although small in absolute magnitude, patients radiated to the left side did have a statistically significant increased risk of MI. These findings support further investigation of techniques to minimize the long-term cardiac risks faced by breast cancer patients.