Room: Exhibit Hall | Forum 4
Purpose: Recentâ€¯publications (Cuculich et al., NEJM,2017 & Robinson et al., Circulation,2019) have â€¯shown â€¯promising outcomes of non-invasive cardiac radio-ablation â€¯using â€¯single-fraction-SBRTâ€¯to treat patients with refractory, life-threatening, Ventricular Tachycardia (VT). In this treatment-planning study we attempted to optimize and standardize the Volumetric Modulated Arc Therapy (VMAT) planning technique to achieve better dose conformity and greater Non-Target Heart-Tissue (NTHT) sparing.
Methods: Thirteen patients were selected who previously received single-fraction-SBRT as part of a phase_I/II_clinical trial (NCT02919618). Doses of 35Gy to ITV and 25Gy to PTV were delivered by 3 non-coplanar VMAT arcs of 6MV_FFF on a Varian Edge linear accelerator. Patients were retrospectively re-planned using 10 optimized VMAT arcs (10arcs_plans) with unique collimator angles using 6MV_FFF on Halcyon (Halc_6x), and 6MV_FFF (Edge_6x) and 10MV_FFF (Edge_10x) on Edge. Optimization goals were to improve dose conformity while maximizing the NTHT sparing and meeting the protocol constraints of stomach, esophagus and colon (OARs). 10arcs_plans were compared against the clinical plans.
Results: Optimized 10arcs_plans improved the dose conformity (Clin:1.14Â±0.12 vs. Halc_6x:1.08Â±0.05, Edge_6x:1.02Â±0.04, Edge_10x:1.03Â±0.03) and reduced gradient measure (Clin:2.01Â±0.47 vs. Halc_6x:1.79Â±0.28cm, Edge_6x:1.76Â±0.47cm, Edge_10x:1.55Â±0.26cm) while achieving adequate V95%Rx coverage for the ITV (clin:99.7Â±0.4% vs Halc_6x:99.6Â±0.4%, Edge_6x:99.3Â±0.8%, Edge_10x:98.7Â±2.0%) and the PTV (Clin:99.7Â±0.4% vs. Halc_6x:97.2Â±1.8%, Edge_6x:96.8Â±2.2%, Edge_10x:97.1Â±1.8%). 10arcs_plans reduced the NTHT-mean-dose (Clin:6.2Â±2.1Gy vs. Halc_6x:3.9Â±1.0Gy, Edge_6x:3.7Â±1.1Gy, Edge_10x:4.0Â±1.1Gy) and the V20Gy (Halc_6x:35cc, Edge_6x:43cc, Edge_10x:38cc), V10Gy (Helc_6x:137cc, Edge_6x:159cc, Edge_10x:141cc) and V5Gy (Helc_6x: 219.0cc, Edge_6x:247cc, Edge_10x:206cc). All VMAT plans met the protocol objectives for the other OARs. For the 10arcs_plans the MU-ratio was increased from 2.66Â±0.26 to 3.74Â±0.32 for Halc_6x and 3.49Â±0.38 for the Edge_6x and 2.92Â±0.30 for the Edge_10x.
Conclusion: All 10arcs_plans provided better dose conformity and gradient while allowing greater sparing of NTHT from doses associated with increased risk for complications. Coplanar plans on a ring gantry system such as Halcyon appear clinically similar to Edge plans.