MRI-Directed EP-Guided Noninvasive Cardiac Radioablation (ENCORE) for Treatment of Ventricular Tachycardia (VT)

Abstract

Early reports of ENCORE using a conventional Linac (CL) with an ITV-approach and kV-IGRT have demonstrated promising preliminary efficacy for treatment of VT. MRI provides superior cardiac tissue visualization compared with kV imaging, and real-time imaging may allow for reduced treatment volumes, thereby improving the therapeutic ratio. We explore the feasibility of using a Linac-based MR-IGRT system (MRL) for ENCORE, with the goal of increasing the precision of therapy and mitigating the effects of respiratory motion. An MRL is currently under installation at our institution, and is otherwise identical to an existing Co60-based system already in clinical use for three years. Both systems have a 0.35-T field, 27-cm2 maximum field size, 50-cm field of view, and real-time imaging capability at 4 frames per second (fps). The MRL produces a flattening-filter-free 6-MV beam at 600 cGy/min, modulated by a doubly-focused, double-stack, multileaf collimator with an effective leaf width of 4 mm. The MRL was evaluated for use in MR-ENCORE along the following criteria: 1) image quality for volumetric and cine scans; 2) heart tracking for breathing-based beam control; 3) image distortion due to presence of implanted cardiac devices (ICD); 4) plan quality in comparison to 5 previously delivered CL IMRT and VMAT plans; 5) delivery time. Imaging quality was adequate for both visualizing cardiac tissue and tracking heart motion with breathing. Minimal distortion (∼0.5 mm) was observed around a sample ICD lead, with null field lines produced by the ICD ending 13 cm from the device. MRL plan quality was comparable to CL VMAT/IMRT plans, with generally greater coverage of the GTV, and improved sparing of uninvolved heart tissue (see Table 1). Average calculated (using known gantry and MLC leaf speeds) delivery time for the coplanar MRL plans was 20.8 min vs. 24.1 min actual delivery time for the predominantly non-coplanar CL. The MRL has the potential to deliver at least equivalent, if not superior, ENCORE plans compared with CL. Furthermore, the superior soft tissue contrast offers additional benefits of potentially narrower setup margins, and thus greater sparing of normal tissues. Additionally, patients may be monitored via both real-time imaging and the cardiac monitoring tools already integrated with the MRL. Coupled with the system’s efficient delivery and clinically-acceptable dose distributions, we have demonstrated its potential for effectively performing non-invasive cardiac ablation.Abstract 266; Table 1Pt 1Pt 2Pt 3Pt 4Pt 5%Vol PTV at 100Rx%CL99.895.096.185.697.6MRL99.098.095.188.498.0Vol Uninvolved Heart at 22 Gy (cc)CL48.815.043.238.020.1MRL45.57.541.029.115.5 Open table in a new tab