Abstract PD12-10 : Impact of Breast Irradiation on Long-Term Risk of Cardiac Events Following Breast-Conserving Surgery for DCIS: A Population-Based Analysis

Abstract Background: Most individuals diagnosed with ductal carcinoma in situ (DCIS) undergo breast-conserving surgery (BCS) followed by whole-breast radiotherapy (RT). Historically, there have been concerns about increased cardiac risk from breast RT, particularly for left-sided lesions. With the implementation of modern RT techniques and cardiac avoidance protocols, the long-term impact of RT on cardiac outcomes in DCIS remains uncertain. Methods: We conducted a retrospective, population-based cohort study of women diagnosed with pure DCIS in Ontario from 1994 to 2014 who underwent BCS with or without RT. Clinical and pathological characteristics, comorbidity burden (Elixhauser index), socioeconomic status, and treatment variables including laterality and RT receipt were obtained from the Ontario DCIS database and linked administrative datasets. Pre-existing cardiovascular disease and subsequent cardiac events were identified using hospital discharge data. Primary outcomes included acute myocardial infarction (AMI), coronary revascularization (Percutaneous Coronary Intervention (PCI) or Coronary Artery Bypass Grafting (CABG)), death due to ischemic heart disease (IHD), and a composite of these events. Fine-Gray competing risk regression was used to estimate the impact of RT, adjusting for age and baseline cardiac disease. Cumulative incidence functions (CIFs) were compared using Gray’s test. Results: Among 3,009 women with pure DCIS treated with BCS, 1,562 (52%) received RT. Median age at diagnosis was 56 years in the RT group and 62 years in the non-RT group. Median follow-up was 22.4 years for those who received RT and 21.2 years for those who did not. On multivariable analyses adjusting for age and history of heart disease, RT was not associated with a significant increase in the risk of AMI (HR 1.13, 95% CI 0.84-1.53, p=0.43), coronary revascularization (HR 1.33, 95% CI 0.95-1.88, p=0.10), death due to IHD (HR 0.89, 95% CI 0.51-1.55, p=0.68), or the composite outcome (HR 1.09, 95% CI 0.85-1.41, p=0.49). At 20 years, the cumulative incidence of AMI was 5.0% with RT vs 5.9% without RT (p=0.39); coronary revascularization was 4.6% vs 4.0% (p=0.26); and death due to IHD was 1.7% vs 3.7% (p=0.0006). The 20-year cumulative incidence of the composite cardiac outcome was 7.1% with RT vs 7.7% without RT (p=0.04). When stratified by laterality, there were no significant differences in 20-year incidence of AMI (4.6% left vs 5.4% right, p=0.68), IHD death (1.6% vs 1.7%, p=0.85), or the composite event (7.1% vs 7.0%, p=0.95). Conclusions: In this large, population-based cohort of women with pure DCIS treated with BCS, whole-breast RT, including to the left breast, was not associated with increased risk of long-term cardiac events after over two decades of follow-up. These findings support the cardiac safety of contemporary breast RT techniques in the management of DCIS and should be considered when weighing the risks and benefits of adjuvant RT. Citation Format: E. Hahn, R. Sutradhar, F. Johri, D. Rodin, K. J. Jerzak, L. Nguyen, S. Trebinjac, C. Fong, L. Paszat, E. Rakovitch. Impact of Breast Irradiation on Long-Term Risk of Cardiac Events Following Breast-Conserving Surgery for DCIS: A Population-Based Analysis abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PD12-10.