Abstract Body (Do not enter title and authors here): Background: Intramural septal substrate represents a major challenge in patients undergoing Ventricular Tachycardia (VT) ablation because the septal thickness could make difficult for radiofrequency to achieve transmural lesions. Although novel approaches have been proposed to treat septal VT, about half of patients experienced VT recurrence and the risk of peri-procedural complications including Atrioventricular Block (AVB) remained high. Stereotactic Arrhythmia Ablation (STAR) was introduced to treat VT refractory to catheter ablation nevertheless no data are available in the setting of septal VT.

Objectives: to evaluate the arrhythmic burden in patients with septal VT treated with Magnetic Resonance Imaging guided STAR (MRgSTAR).

Methods: We enrolled consecutive patients with septal VT substrate. The therapy target was achieved combining anatomical/functional and electrophysiological data (Fig. 1). Patients were treated with a single fraction of 25Gy adopting MRgSTAR (Fig. 2). All patients were clinically followed-up and all implantable cardiac device were remote monitored. The efficacy outcome included recurrences of any sustained VT beyond the 6-weeks blanking-period after MRgSTAR. The safety outcome was the incidence of adverse events and AVB.

Results: We included 11 patients with septal substrate VT [median age:68yrs (IQR:64.5–78yrs); 100% male]. Clinical presentation was electrical storm in 81.8% of patients. No complications occurred after MRgSTAR and six (54.5%) patients were discharged the same day of treatment. During a mean follow-up of 12±6 months, the efficacy outcome occurred in three (27.3%) cases. A significative reduction of Implantable Cardioverter Defibrillator (ICD) therapy (23.6 before MRgSTAR vs 1.7 after MRgSTAR, p-value:<0.001) was observed (Fig. 3). Left ventricular ejection fraction increased significantly after treatment [38%(IQR:33.5-42%) before MRgSTAR vs 43.8%(IQR:35–47%) after MRgSTAR; p-value:0.04]. No adverse effects were observed in ICD system performance, lead thresholds, or lead impedances at any point after treatment. In the seven patients with preserved AV conduction, no AVB or left/right branch block were reported after STAR. No difference in the percentage of ventricular pacing was described before and after STAR [3.5% (IQR:2.75-4%) before STAR vs 4.5% (IQR: 3.25–5.5%) after STAR; p-value: 0.24].

Conclusions: MRgSTAR represents a safe and effective strategy for the treatment of septal VT.