Association Between Sinoatrial Node Radiation Dose and New-Onset ECG Abnormalities After Breast Cancer Intensity-Modulated Radiotherapy.

PURPOSE: To investigate the association between cardiac conduction system dosimetry, specifically the sinoatrial node (SAN), for new-onset electrocardiogram (ECG) abnormalities in breast cancer (BC) patients undergoing intensity-modulated radiotherapy (IMRT). METHODS: We retrospectively reviewed 91 BC patients treated with postoperative IMRT between October 2023 and May 2024. Cardiac substructures, including the SAN, atrioventricular node (AVN), ventricles, and coronary arteries, were retrospectively delineated according to previously published contouring guidelines for dosimetric analysis. The cumulative incidence of new-onset ECG abnormalities was estimated using the Kaplan-Meier method. Independent dosimetric factors associated with new-onset ECG abnormalities were identified using multivariate Cox proportional hazards regression analysis. RESULTS: At a median follow-up of 9.0 months, the estimated cumulative incidences of new-onset ECG abnormalities at 3, 6, and 12 months based on Kaplan-Meier analysis were 17.6%, 38.3%, and 76.7%, respectively. Multivariate analysis identified mean heart dose (MHD) and the SAN volume receiving ≥ 5 Gy (SAN V5) as potent independent factors associated with new-onset ECG abnormalities. Each 1-Gy increase in MHD was associated with a higher risk of new-onset ECG abnormalities (HR = 1.28; 95% CI, 1.09-1.50; P = 0.002). An exploratory SAN V5 threshold of ≥3% was associated with an increased risk of new-onset ECG abnormalities (HR = 1.71; P = 0.003). CONCLUSION: In this exploratory retrospective cohort, SAN radiation exposure was associated with new-onset ECG abnormalities post-IMRT. These findings suggest that cardiac substructure dosimetry may provide additional information beyond conventional mean heart dose assessment.