Rhythm-RT: A New Tool for Target Volume Delineation in Cardiac SBRT

Abstract

<h3>Purpose/Objective(s)</h3> Nearly half of all patients with ventricular tachycardia (VT) become refractory to standard-of-care anti-arrhythmic medications and catheter ablation. Stereotactic body radiotherapy (SBRT) has emerged as a well-tolerated and effective new treatment for VT. Since arrhythmogenic lesions are undetectable on CT scans, the practice of SBRT target volume delineation is not yet formalized. Many single-institution protocols involve only verbal discussion between electrophysiologists and radiation oncologists or manual re-orientation of surface renderings from electrophysiologic (EP) studies to correlate with the planes on CT scan. We present a novel software, "Rhythm-RT", that synthesizes data from EP studies and heart contours from radiation planning to automatically generate a preliminary target volume. <h3>Materials/Methods</h3> Along with DICOM files containing contours, Rhythm-RT can process electroanatomic mapping (EAM) data from CARTO (Biosense Webster, Irvine, CA) and EnSite Precision (Abbott, St. Paul, MN) as well as ablation navigation data from EnSite Precision. All input data are plotted in the reference coordinate system of the CT simulation scan. The user is able to translate and rotate imported EP spatial information for co-registration with contoured cardiac substructures. Rhythm-RT then identifies the cardiac substructure closest to the sites of low voltage from EAM or inadequate previous ablation on which to generate an SBRT gross target volume (GTV), adapted to the myocardial shape and transmural thickness of the cardiac substructure. The GTV is exported as a DICOM file which can be integrated with other contoured structures in the original radiation planning system. <h3>Results</h3> From October 2019 to February 2021, 7 males and 1 female with VT (53–84 years of age, ejection fraction 10–65%, and 0–7 previous catheter ablations) were treated using Rhythm-RT to guide cardiac SBRT planning. Compared to existing methods of delineating GTV crudely by free-hand, Rhythm-RT visualizes EP spatial information superimposed on heart contours. The area of myocardium selected as GTV, based on proximity to relevant EP signals, is computationally derived rather than inferred and liable to human error. Final GTV modification with the help of an experienced electrophysiologist is still recommended. When structural heart disease is multifocal, he/she may provide insight on which areas are more likely to cause arrhythmia. Adjustment of the GTV boundary may also be informed by direct observations during EP procedures that are not reflected in procedure equipment logs. <h3>Conclusion</h3> Rhythm-RT is a software that improves the accuracy and efficiency of target volume delineation in cardiac SBRT. Additional patient enrollment on our institutional protocol utilizing this technology is ongoing, along with outcomes analysis and expansion of input data versatility to include all EP and radiation planning systems.