A motion analysis of cardiac substructures for guiding stereotactic arrhythmia radiotherapy motion management

Stereotactic arrhythmia radiotherapy (STAR) has recently emerged as a novel noninvasive treatment option for critically ill and drug-refractory VT patients who cannot be treated or re-treated by catheter ablation. However, STAR requires precise management of cardiorespiratory motion to minimize radiation toxicity to the healthy heart and nearby organs-at-risk (OARs) and to ensure the radiation precision to deliver the prescribed dose to the VT target.

To investigate the characteristics of cardiac motion of VT patients to facilitate cardiac motion management for STAR treatments.

Breath-hold cardiac 4DCTs (c4DCT) of 18 patients previously treated with STAR were analyzed retrospectively. Each c4DCT contained ten 3DCTs corresponding to 10 phases of an entire cardiac motion cycle. For each c4DCT, a group-wise deformable image registration (DIR) method was used to register all ten 3DCTs, resulting in ten 3D deformation vector fields (DVFs) and an average position 3DCT. The DVFs were computed from each phase to the average position 3DCT instead of between pairs of phases. Metal artifacts caused by the Implantable Cardioverter-Defibrillator (ICD) leads were reduced using a diffusion procedure before DIR. The heart chambers were segmented on the average position 3DCT using an AI segmentation tool, followed by manual evaluation and correction. Cardiac motion characteristics (magnitude and direction) were investigated temporally over the 10 cardiac phases and spatially for the outer myocardium walls of the heart and per chamber, AV (atrium-ventricle) valves, septa (the muscular walls dividing the chambers), and STAR targets.

The motion magnitude maximum over cardiac phases was computed by taking each voxel's maximum motion magnitude in 10 phases, referring to the average position. The cardiac motion magnitudes were found to be strongly patient-specific. The maximum, 99^th percentile, and 95^th percentile of the motion maximum for the myocardium wall among patients ranged from 8.7 to 17.8 mm, 4.9 to 11.9 mm, and 2.9 to 8.9 mm, respectively. The same metrics for STAR target motion ranged from 2.9 to 11.6 mm, 2.6 to 6.4 mm, and 2.4 to 6.0 mm, respectively. Z-tests showed a significant difference between the maximum motion magnitude of the myocardium and the STAR target, while there was no significant difference between the mean motion magnitude of the myocardium and the STAR target. The 95^th percentiles of the myocardium motion magnitudes were < 5 mm for 16 out of 18 patients. The largest motion appeared near the ventricle-atrium (AV) valves. All cardiac structures have their largest displacements from the average position at the end of systole and diastole.

The cardiac motion characteristics (magnitude, direction, and spatial distribution) of STAR patients provided in this study would further facilitate cardiac motion management.