347 - Longitudinal Trends in Myocardial Metabolism following Non-Invasive Cardiac Radioablation for Ventricular Tachycardia Using PET-CT

G. M. Walls¹, G. D. Hugo², R. Ghadban¹, A. Javaheri³, K. M. S. Moore¹, N. Knutson², D. H. Cooper¹, S. Rentschler¹, P. Samson¹, C. G. Robinson¹, P. Cuculich¹, and C. Bergom³; ¹Washington University in St Louis, St Louis, MO, ²Washington University School of Medicine in St. Louis, Department of Radiation Oncology, St. Louis, MO, ³Washington University School of Medicine, St. Louis, MO

Purpose/Objective(s): Ventricular tachycardia (VT) is a life-threatening rhythm disturbance of the heart that has been shown to be amenable to treatment with SBRT, known as non-invasive cardiac radioablation (CRA). The metabolic impact of directly targeting arrhythmogenic regions of scarred left ventricle (LV) is unknown. Herein the relationship between RT dose and ¹⁸FDG uptake in LV segments following CRA was interrogated. Materials/

Methods: Patients in the ENCORE–VT trial received a 25 Gy single-fraction CRA to an electrophysiology-guided gross target volume (GTV), planned on 4D-CT. Patients underwent PET-CT, troponin–I and MRI at baseline, day 3 (D3) & day 90 (D90). Auto-segmentation of LV into the AHA 17-segment model was performed on planning CTs & PET-CTs, allowing median RT dose and standardized uptake value (SUV) to be extracted. Change in segment median SUV was correlated with dose, clinical outcomes (ICD shock, death) & biomarkers.
Results: 19 patients received CRA. Median VT episodes were reduced from 119 in the 6 months before compared to 3 in the six months after CRA (p<0.001). In the first 90 days, 2/19 patients (10.5%) developed a treatment-related SAE. Overall survival was 89% at 6 months. Targeted LV segments had lower baseline SUV than non-targeted segments (p<0.001). Median SUV of targeted segments at baseline, D3 & D90 were 1.43, 1.67 & 2.52 (p=0.012). There were time-dependent decreases in troponin–I, and from as early as D3 (p=0.332). At D3, greater SUV increases were observed in cardiac segments that received lower doses (0–10Gy SUV +37.2% vs 20–40Gy SUV +8.0%, p=0.004). At D90, the opposite trend was seen; greater SUV increases were seen in segments that received higher doses (20–40Gy SUV +106.7% vs 0–10Gy SUV +79.0%, p=0.012). At D3, a =20% SUV increase in targeted segments (n=7/19) was associated with improved survival (p=0.006). At D90, increased median whole myocardium SUV was weakly associated with improved LVEF (R²=0.355, p=0.158). In logistic regression models, SUV values, together with cardiomyopathy type, VT storm, and amiodarone use predicted shocks (AUC 0.828, p=0.027) and death (AUC 0.847, p=0.016).
Conclusion: Metabolic activity of the GTV increased after CRA. Late (D90) SUV increases were observed in targeted segments, consistent with VT burden reduction. Which cell populations (e.g. cardiomyocytes, immune cells, fibroblasts) have become more metabolically active remains to be determined. Unexpectedly, early SUV changes (D3) were observed in segments in the low dose bath outside the GTV, and were associated with both reduced troponin–I and improved survival. Further longitudinal studies with PET-CT after CRA will define how SUV dynamics can be optimally incorporated as a biomarker in practice, and the potential therapeutic functional impact of low-dose RT in the setting of cardiomyopathy warrants further investigation.