Quantifying cardiac deformable image registration accuracy and its dosimetric variability for 4D dose accumulation in stereotactic arrhythmia radioablation

Objective Stereotactic arrhythmia radioablation (STAR) offers a non-invasive treatment option for refractory tachycardia; however, precise dose delivery remains challenging due to the complexity of cardiorespiratory motion. This study evaluated the geometric and dosimetric performance of multiple deformable image registration (DIR) algorithms using ECG-gated four-dimensional CT (ECG-4DCT) in both virtual phantom and clinical datasets. Approach ECG-4DCT data from the XCAT phantom and 20 patients were analyzed across 10 cardiac phases using six DIR algorithms, with a seventh algorithm, TransMorph, additionally evaluated on the clinical datasets. Registration accuracy was assessed using the Dice similarity coefficient (DSC), Hausdorff distance (HD95), and average surface distance (ASD), while dosimetric accuracy was evaluated using dose-volume histogram metrics and γ analysis. Four-dimensional dynamic dose (4DDD) variability was quantified using the coefficient of variation (CV) and maximum pairwise absolute dose difference (MPADD). Main results Registration accuracy was lowest between the end-systolic and end-diastolic phases in both phantom and clinical datasets. In the phantom study, MIM achieved the highest γ passing rate (89.6% at 1%/1 mm) and the slightest deviation from the reference, with differences of -0.02 Gy in D95 and -0.2% in V25. In the clinical datasets, patients without metallic implants exhibited reduced geometric accuracy and increased 4DDD variability (mean CVs of 0.01 for V25 and 0.02 for D95; MPADDs up to 11.3% and 1.15 Gy). TransMorph achieved the highest geometric accuracy, with mean DSC values of 0.86 in patients without implants and 0.89 in those with implants; however, this improved geometry was accompanied by steeper deformation gradients and pronounced localized dose discrepancies. Significance Current DIR algorithms remain limited in capturing complex cardiac motion for STAR. Geometric accuracy alone is insufficient to ensure physiologically plausible deformation, underscoring the need for cardiac-specific, physiology-constrained DIR frameworks to enable robust and clinically reliable 4DDD evaluation.