Dosimetric Performance Analysis of Pediatric Based Halcyon™ Volumetric Arc Therapy-Based Lung-Sparing Total Body Irradiation.

IntroductionHalcyon™ O-ring linear accelerators (linacs) provide a robust and efficient radiotherapy option for clinics. Interest in volumetric arc therapy (VMAT) based total body irradiation (TBI) has grown, with most current VMAT TBI protocols employing C-arm linacs; however, the feasibility of Halcyon-based VMAT TBI has been recently hypothesized. This study investigates the planning feasibility and dosimetric quality of VMAT-based, lung-sparing TBI delivered by Halcyon linac to pediatric patients.Methods10 pediatric patients (94 - 134 cm in height) previously treated to 12 Gy TBI through traditional 3D planning techniques were retrospectively replanned on a Halcyon linac. 2-3 VMAT plans containing 4-6 isocenters each with 2-2.5 arcs were created to treat the head, abdomen, and legs, with 1-2 static-gantry treatment matching areas being used between VMAT planning regions.ResultsLung doses were significantly reduced below the clinically significant level of 8 Gy, achieving mean doses of 6.66 ± 0.52 Gy (55.5% ± 4.3%). Target coverage was achieved with D90% of 77.5% ± 12.5% and homogeneity indexes of 0.92 ± 0.1. Planning time took 5-6 hours per patient: 1.5 hours for contouring and field placement and 3.5-4.5 hours for plan optimization and dose calculations. Using a dose rate of 800 MU/min field delivery time took an average of 20.8 ± 7 minutes with average portal dosimetry quality assurance passing rates of 99.6% and 99.9% for 2%/2mm and 3%/3mm gamma analysis metrics.ConclusionVMAT lung sparing TBI via Halcyon linacs is feasible and dosimetrically acceptable, with greatest applicability to smaller patients (<100 cm). Planning and setup complexity increases dramatically for larger patients (>100 cm) due to the smaller maximum field sizes (28 x 28 cm2) achievable. Future developments in software-based automations for target delineation, multiple field matching, and plan optimization could allow for more practicable treatment of larger patients.