Author: Chonthicha Bunchuailuea, David Geeler, Martin Sabel, Claas Wessels 👨🔬
Affiliation: Varian Medical Systems 🌍
Purpose: Elongated treatment sites, such as craniospinal treatments, pose multiple challenges in today’s practices, requiring multiple CBCTs for position verifications per fraction, which might increase the risk of toxicities, especially for pediatrics under general anesthesia. A single field, by moving the couch longitudinally, could reduce the burden of such treatments; however, it is unclear if today’s treatment devices are capable of this delivery technique.
Methods: Several custom XML beams were programmed in Matlab with varying combinations of the delivery parameters, including number of MUs, dynamic longitudinal travel distance of 60cm and 100cm, and number of control points (CP) in the range of 10 to 300 CPs per rotation for a TrueBeam system (Varian Medical Systems, USA) and a Halcyon system (Varian) for three complete consecutive gantry rotations. The resulting log files were exported to calculate the differences between planned and actual values in Matlab. The data was plotted and statistically analyzed by determining the differences’ mean±standard deviation, minimum (min), and maximal (max) values. Furthermore, the couch's gravitational sag was measured using the Absolute Tracker (Leica, Switzerland) and the T-Mac (Leica) 6DOF sensor and averaged over six runs.
Results: The TrueBeam system's overall mean couch position difference was -0.02mm±0.07mm with a min/max of -0.32mm/0.15mm. The gantry angle had a mean difference of 0.02°±0.06° with a min/max of -0.1°/0.1°. The couch sag for a pediatric weight was 1.82mm. The couch's mean longitudinal difference for Halcyon plans was -0.02mm±0.06mm with a min/max of -0.32mm/-0.04mm. The gantry angle's mean difference was 0.01±0.03° with a min/max of -0.05°/0.05°. The couch sag was determined to be 1.7mm.
Conclusion: The synchronization of gantry rotation with the dynamic longitudinal couch motion is sufficiently accurate, demonstrating differences less than 0.5mm and 0.1°. However, the gravitational sag's hard-coded compensation of the couch overcorrects the vertical position for pediatric weights.