Phantom study of stereotactic radioablation for ventricular tachycardia (STRA-MI-VT) using Cyberknife Synchrony Respiratory Tracking System with a single fiducial marker

Purpose: Within the STRA-MI-VT phase Ib/II trial (NCT04066517), the aim of this phantom study was to explore the feasibility of Cyberknife treatments on cardiac lesions by tracking as a single marker the lead tip of an implantable cardioverter defibrillator. The residual displacement of the lesion during the tracking was studied, planning margins were found and the dosimetric accuracy of the treatment was checked. Materials and methods: A lead was inserted into a phantom (EasyCube phantom, Sun Nuclear Co, USA) and then placed on the translating ExacTrac Gating System (BrainLAB AG, Germany). The phantom was rotated, a virtual lesion was identified and its displacement during the tracking was studied. Two plans were compared, calculated on the unrotated volume and on the envelope of the unrotated and the rotated volumes. The plans were delivered using the Cyberknife System (Accuray Inc, USA) and their dosimetric accuracy verified by gamma analysis with gafchromic films. Results: The residual margin increases enhancing the distance between the lead and the lesion. It is 4 mm for distance 0 cm and 5 mm for distance 5 cm. The coverage is reduced by 3.8% (interquartile range 2.5%–4.7%) when the dose is prescribed on the unrotated volume. All treatment plans are accurate and 3% 3 mm gamma analysis results are greater than 94%. Conclusions: Results showed that tracking with a single marker is feasible considering adequate residual planning margins. The volumes could be further reduced by using additional markers, for example by placing them on the patient's skin.