This study aims to investigate whether patients with breast cancer and a history of cardiovascular diseases (CADs) are at an increased incidence of acute coronary syndrome (ACS) after receiving radiation therapy (RT). In Taiwan, 5828 patients who had a history of CAD were newly diagnosed of breast cancer and received mastectomy between 1999 and 2009. Among these patients, 1851 also received RT. The study cohort was prospectively followed to the end of 2010 for estimating the incidence of ACS in association with exposure to RT. A Cox proportional hazard model that was adjusted for covariates was used to estimate the hazard ratio (HR) of ACS. Over the study period, the incident rates of ACS for RT and control patients were estimated at 1.51 and 1.77 per 100 person-years, respectively. Covariate-adjusted regression analysis indicated that the hazard of ACS significantly increased in RT patients at an adjusted HR of 1.48 [95 % confidence interval (CI) 1.18–1.87]. Both hypertension and diabetes significantly increased the hazard of ACS in this patient cohort, with adjusted HRs of 3.31 (95 % CI 1.94–5.66) and 1.50 (95 % CI 1.19–1.89), respectively. This 12-year follow-up study suggested excess of ACS events in association with RT exposure in patients with breast cancer who had a higher cardiovascular risk. In consideration of the benefit associated with RT, intensive cardiac care should be given to patients with breast cancer and high cardiovascular risk.
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