Volumetric Modulated Arc Therapy Planning for Craniospinal Irradiation With a New O-ring Linac

This study aims to determine the feasibility of using a new O-ring linear accelerator (Halcyon, Varian Medical Systems, CA, USA) to perform treatment planning using volumetric modulated arc therapy (VMAT) for craniospinal irradiation (CSI). A 20-year-old male patient with leukemia was selected. The planning target volume (PTV) was contoured to include the entire contents of the brain and spinal canal. The PTV margin was 10 mm applied to the clinical target volume (CTV). VMAT (RapidArc, Varian Medical Systems, CA, USA) planning was performed using four isocenter with five arcs, two full rotation arcs to cover the brain and upper part of the spinal cord, and one full rotation arc for the lower part of the spinal cord. The plan was created using the auto-feathering photon optimizer calculation of the planning system. The conformity index (CI) and heterogeneity index (HI) as well as dose-volume histograms of organs at risk (OAR) were evaluated. The patient position of ±3.0 mm in the craniocaudal direction was moved in to simulate the effect of treatment inaccuracy. The total treatment time was also measured. The CI and HI were 1.09 and 8.44, respectively. The mean dose (PTV) was 105.5%, and the mean dose (OARs) was lower than the planning dose constraints. Simulations with a patient position shift of ±3.0 mm resulted in an error of less than ±10.0% of the planned dose to the spinal cord. The total treatment time was within 15 minutes. VMAT planning for CSI with Halcyon achieved high conformality, uniform dose distribution, low dose to the surrounding normal tissues, and reduced treatment time.