Abstract
BACKGROUND: Postoperative radiation therapy (RT) after lumpectomy reduces the risk of locoregional recurrence in ductal carcinoma in situ (DCIS). However, the potential association between RT and ischemic heart disease (IHD) remains uncertain. This nationwide cohort study evaluated the long-term impact of postoperative RT on IHD risk and overall survival (OS) in women with DCIS using real-world data from the Korean National Health Insurance Service (NHIS). METHODS: Women diagnosed with DCIS who underwent breast-conserving surgery between 2003 and 2020 were identified from the NHIS claims database. Patients with invasive breast cancer, age under 20 years, a prior history of IHD, or missing smoking or body mass index (BMI) data were excluded. Multivariable Cox regression was performed to assess the association between postoperative RT, IHD incidence, and OS, adjusting for key cardiovascular risk factors. RESULTS: Among 4633 eligible patients (RT, 2778; no RT, 1855), the median follow-up duration was 86.1 months, and baseline characteristics were well balanced between groups without major differences in cardiovascular risk factors. A total of 126 patients (3.4%) developed IHD, with a 10-year cumulative incidence of 4.7%. Older age, hypertension, and hyperlipidemia were independent risk factors for IHD, whereas postoperative RT was not significantly associated with increased IHD risk (hazard ratio [HR] = 1.07, 95% confidence interval [CI] = 0.77-1.48; p = 0.690). The 10-year OS rate was 98.0%, and postoperative RT remained an independent predictor of improved survival (HR = 0.47, 95% CI = 0.28-0.79; p = 0.004). CONCLUSIONS: Postoperative RT did not increase the long-term risk of IHD but was associated with improved OS in patients with DCIS. These findings provide population-based evidence supporting the cardiac safety and oncologic efficacy of postoperative RT, while recognizing that unmeasured differences in health behavior or medical care utilization could have contributed to the observed survival benefit.