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) 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.