2734 - Early Ventricular Tachycardia Suppression after Cardiac Radioablation using SABR Suggests Electrical Conduction Remodeling Rather Than Fibrosis: Clinical Timeline Outcomes and Dosimetric Correlates

Purpose/Objective(s):

Utilizing stereotactic ablative radiotherapy (SABR) for refractory ventricular tachycardia (VT) in patients who are not candidates for further catheter ablation represents an evolving therapeutic approach; however, many aspects of this treatment remain uncertain. Historically, cardiac radioablation (CRA) was thought to act primarily through radiation-induced fibrosis. More recent evidence suggests an alternative mechanism of action involving electrical modulation of myocardial conductivity via altered sodium-channel and connexin expression (1). Accordingly, rapid VT suppression (=1 month) following SABR may reflect conduction remodeling rather than delayed fibrosis formation, which traditionally requires a longer latency period. Such observations may support future consideration of earlier or emergent indications for CRA.

Furthermore, understanding the influence of target location and dosimetric parameters may be critical for refining intracardiac dose constraints and optimizing treatment planning. We conducted a retrospective single-institution analysis to characterize demographic, dosimetric, and clinical outcomes in patients treated with CRA.

Materials/Methods:

Between 2021 and 2026, twenty patients underwent CRA using SABR, each receiving a prescribed dose of 25 Gy in a single fraction, with an American Heart Association 17-segment model–based target delineation approach. Clinical records and treatment-planning data were retrospectively reviewed. Dosimetric parameters, including clinical target volume (CTV), planning target volume (PTV), heart V16, and maximum point dose (Dmax), were collected and analyzed in relation to time to VT suppression and subsequent recurrence based on implantable cardioverter-defibrillator (ICD) interrogation.

Results:

Rapid VT suppression (=1 month) was achieved in 11 patients (55%), while 2 patients (10%) demonstrated delayed suppression (>1 month), and 7 patients (35%) had no response. Median CTV was 25.6 cc (IQR 17–31.5), PTV 73 cc (IQR 41–100), V16 118 cc (IQR 48–170), and Dmax 29 Gy (range 26–33).

The most frequently treated regions were the basal anterior (segment 1, 40%) and basal anteroseptal (segment 2, 35%), with a median of three left ventricular segments treated per patient (range 2–5).

Conclusion:

Although this retrospective series is limited by a small sample size, it provides preliminary observations on treatment response patterns. Rapid VT suppression (=1 month) in 55% of patients supports a mechanism mediated by modulation of myocardial conductivity. The most commonly treated segments (1 and 2) were near the mitral valve, underscoring the need for careful attention to intracardiac dose constraints and long-term monitoring for potential valvular effects. Involvement of multiple left ventricular segments may indicate limited target precision, contributing to larger PTVs and emphasizing the need for improved targeting method.

Reference:

1. Zhang et al., Nat Commun, 2021