Prospective Clinical Outcomes of Cardiac Stereotactic Body Radiotherapy for Refractory Ventricular Tachycardia　― Insights From Dose Distribution in Survivors ―

Background: Clinical outcome determinants in cardiac stereotactic body radiotherapy (cardiac-SBRT)/stereotactic arrhythmia radioablation (STAR), a minimally invasive therapy for refractory ventricular tachycardia (VT), remain unclear. We report the final analysis of Japan’s first prospective study following a previously published interim report. Methods and Results: Between 2019 and 2024, 8 patients with recurrent VT refractory to medical therapy and unsuitable for or refractory to catheter ablation were enrolled. Primary endpoints were safety and VT suppression. Exploratory analyses examined irradiation distribution and autonomic function. Median age was 69 years (75% male), ejection fraction 25%, and median follow-up 2.2 years. Targets were defined using multimodal assessment, with a median planning target volume of 88.9 cc. Treatment-related adverse events included only grade 1 nausea (n=1). Implantable cardioverter-defibrillator shock suppression was 73.6% at 6 months and 76.8% at 1 year. Antitachycardia pacing suppression reached 97.9%, with near-complete suppression in 5 survivors. Analyses remained descriptive and hypothesis-generating. Heart rate variability suggested increased parasympathetic activity, and 123 I-metaiodobenzylguanidine imaging indicated reduced washout rate. Three patients died within 2 years but had advanced preexisting heart failure; irradiation more often involved inferior segments. Survival differences were not explained by cohort-level B-type natriuretic peptide levels or systolic function changes. Conclusions: Cardiac-SBRT/STAR achieved sustained arrhythmia suppression. Heart failure severity, irradiation location, and autonomic function, with their interaction, may be associated with clinical outcomes.