KEY POINTS
- The retrospective cohort included 91 consecutive breast cancer patients receiving postoperative IMRT between October 2023 and May 2024. All had baseline LVEF ≥50%, a normal pre-RT ECG, and no known arrhythmia or ischemic heart disease.
- Patients received approximately 50–50.4 Gy in 25–28 fractions, with regional treatment according to indication. Cardiac conduction structures were delineated retrospectively and were not used during treatment-plan optimization.
- Median mean heart dose was 6.47 Gy, while median sinoatrial node Dmean was 4.52 Gy, Dmax 6.27 Gy, and V5 39.4%. The study illustrates how whole-heart metrics may not accurately reflect exposure of individual conduction structures.
- At a median follow-up of 9 months, Kaplan–Meier estimated cumulative incidences of newly documented ECG abnormalities were 17.6% at 3 months, 38.3% at 6 months, and 76.7% at 12 months. The endpoint included rhythm, conduction, and repolarization abnormalities rather than only clinically significant arrhythmias.
- Each 1-Gy increase in mean heart dose was independently associated with greater risk of a new ECG abnormality (HR 1.28, 95% CI 1.09–1.50; p=0.002).
- An exploratory data-derived threshold of sinoatrial node V5 ≥3% was also independently associated with ECG abnormalities (HR 1.71; p=0.003).
- ECG surveillance was not performed at uniform protocol-defined intervals, follow-up was short, and the clinical significance of many ECG changes is uncertain. Current breast RT guidelines do not include validated sinoatrial-node constraints.
CLINICAL TAKEAWAY
The sinoatrial node may represent another relevant cardiac substructure in breast radiotherapy, and mean heart dose alone may not capture all electrophysiologically relevant exposure. The proposed V5 signal is hypothesis-generating and should not yet be adopted as a planning constraint.