Purpose: Adjuvant radiation therapy (RT) for breast cancer is associated with heart disease, although the impact of patient-specific factors on the interaction between cardiac risk and RT is not well-studied in cancer patients. The objective of this study is to compare acute coronary events (ACE) among the general population and women with breast cancer after adjuvant RT. Secondary analysis evaluated whether a healthy lifestyle could protect against RT-related cardiac toxicity. Methods: The National Health Insurance Service-Health Screening Cohort (2002–2013) was used to compare ACE risks among 1015 women with breast cancer and among 8120 women without cancer who were matched according to age, comorbidities, and smoking history. The risk of developing ACE over time while accounting for competing risks from other causes of death was analyzed. Results: During 6.1 ± 3.0 years of follow-up, the 5- and 10-year cumulative incidences of ACE were 5.5% and 11.3%, respectively. The breast cancer survivors who underwent breast radiotherapy and population-based matched sample had similar risks of ACE (hazard ratio: 0.94, 95% confidence interval: 0.69–1.28). However, in the sensitivity analysis, breast cancer survivors had increased risks of ACE if they did not exercise (hazard ratio 2.74, confidence interval: 1.27–5.91) or had a disability (hazard ratio 21.9, confidence interval: 2.50–191.6). Conclusions: In this matched cohort study, the cardiac risk after adjuvant RT increased with decreasing physical activity. The long-term effect of physical activity on ACE is uncertain, but these results can increase physicians' awareness of the approaches to increase exercise participation level among women undergoing RT for breast cancer. Confirmatory studies with individual doses of cardiac radiation and quantification of physical activity and sedentary time are required for validating our results.
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