LAUSR

INTRODUCTION Radiotherapy deliveries with dynamic couch motions that shorten the source-to-axis distance (SAD) on a C-arm linac have the potential to increase treatment efficiency through the increase of the effective dose rate. In this investigation we convert clinically deliverable VMAT and DCA plans for cranial radiosurgery into virtual isocentre plans through implementation of couch trajectories that maintain the target at a shortened SAD throughout treatment. MATERIALS AND METHODS A randomly sampled population of patients treated with cranial radiosurgery from within the last three years were separated into groups with one, two, and three lesions. All plans had a single isocentre (regardless of number of targets), and a single prescription dose. Patient treatment plans were converted from their original delivery at a standard isocentre to a dynamic virtual isocentre in MATLAB. The virtual isocentre plan featured a variable isocentre position based upon the closest achievable source-to-target distance (referred to herein as a virtual source-to-axis distance - vSAD) which avoided collision zones on a TrueBeam STx platform. Apertures were magnified according to the vSAD and monitor units at a given control point were scaled based upon the inverse square law. Doses were calculated for the plans with a virtual isocentre in the Eclipse (v13.6.23) treatment planning system (TPS) and were compared with the clinical plans. Plan metrics (MU, Paddick conformity index, gradient index, and the volume receiving 12 Gy or more), normal brain dose-volume differences, as well as maximum doses received by OARs were assessed. The values were compared between standard and virtual isocentre plans with Wilcoxon Sign Ranked Tests to determine significance. A subset of the plans were mapped to the MAX-HD anthropomorphic phantom which contained an insert housing EBT3 GafChromic™ film and a PTW 31010 microion chamber for dose verification on a linac. RESULTS Delivering plans at a virtual isocentre resulted in an average reduction of 20.9% (p = 3×10-6 ) and 20.6% (p = 3.0×10-6 ) of MUs across all VMAT and all DCA plans, respectively. There was no significant change in OAR max doses received by plans delivered at a virtual isocentre. The low dose wash (1.0 - 2.0 Gy or 5 - 11 % of the prescription dose) was increased (by approximately 20 cc) for plans with three lesions. This was equivalent to a 2.7 - 3.8% volumetric increase in normal tissue receiving the respective dose level when comparing the plan with a virtual isocentre to a plan with a standard isocentre. Gamma pass rates with a 5%/1mm analysis criteria were 96.40% ± 2.90% and 95.07% ± 3.10% for deliveries at standard and virtual isocentre, respectively. Absolute point dose agreements were within -0.36% ± 3.45% and -0.55% ± 3.39% for deliveries at a standard and virtual isocentre, respectively. Potential time savings per arc were found to have linear relationship with the monitor units delivered per arc (savings of 0.009 s/MU with an r2 = 0.866 when fit to plans with a single lesion). CONCLUSIONS Converting clinical plans at standard isocentre to a virtual isocentre design did not show any losses to plan quality while simultaneously improving treatment efficiency through MU reductions. This article is protected by copyright. All rights reserved.