Evaluation of Mid-position based treatment planning for stereotactic radioablation of ventricular tachycardia

Stereotactic arrhythmia radioablation (STAR) has emerged as a treatment for refractory ventricular tachycardia (VT), showing significant arrhythmia reduction and promising safety profile. Challenges remain, such as the optimization of the target volume, aiming at only focusing on the ventricular substrate while sparing organs at risk. This study evaluates the potential dosimetric benefits of the mid-position (MidP) concept compared with Internal Target Volume (ITV) approach for treatment planning of STAR.

All patients were treated in a C-arm LINAC. For each patient, a cardiac-gated four-dimensional computed tomography scan (4D-CTcard) and a respiratory-gated scan (4D-CTrespi) were acquired, and both reconstructed in 10 phases. The cardiac target volume (CardTV) was delineated on the end-of-diastole phase of the 4D-CTcard. For the ITV approach, the CardTV was propagated firstly to the other cardiac phases by deformable registration and secondly to all respiratory phases by rigid image registration. PTV margins were defined at 3 mm. For the MidP approach, the mean time-weighted position of the CardTV was deduced from the DICOM coordinates of the CardTV in the different image frames. MidP-based PTV margins were calculated based on van Herk’s margin recipe considering cardiac and respiratory motions as patient-specific random errors. Treatment plans were optimized such as the prescription isodose of 25 Gy (or 20 Gy for patients for which the left coronaries arteries were nearby the CardTV) encompassed the PTV while respecting organs at risk (OAR) dose constraints. All treatments plans were then normalized such that 95% of the PTV received 100% of the prescription dose.

From October 2021 to July 2024, 16 patients (74.9±8.5 yo, all men) were treated using this multimodal imaging delineation software. The underlying cardiomyopathy included ischemic (n=7), primary dilated (n=7), hypertrophic (n=1) and arrhythmogenic right ventricular cardiomyopathy (n=1). Compared with ITV strategy, the MidP strategy resulted in a mean [min-max] relative PTV volume reduction of 29% [22%, 41%] (p<0.001). The mean [min-max] D95% CardTV coverage was 102% [99%-106%] and 105% [100%-108%] of the prescription dose for MidP and ITV-based plans, respectively. The median heart dose was significantly lower with MidP-based plans with a mean difference of -0.2 Gy (p<0.001). The near-maximum dose (D1%) delivered to left coronary arteries, aorta and stomach were systematically lower with the MidP-based plans.

Compared to ITV based approach, the use of mid-P strategy for treatment planning of STAR leads to significantly smaller PTV and lower surrounding OAR doses while still achieving a clinically acceptable CardTV coverage.