Temporal Sensitivity of Synchronous Proton Pencil Beam Scanning and Dynamic Collimation
Abstract
Purpose
Dynamic collimation is emerging as a next-generation energy-specific collimating technology that can drastically improve the lateral and distal conformity of Pencil Beam Scanning (PBS) proton therapy. Current treatment delivery approaches rely on static step-and-shoot techniques. As such, it was the focus of this work to investigate the temporal sensitivity of spot scanning and collimator synchronous sequencing using a sliding window delivery approach to improve treatment efficiency.
Methods
A time-based Monte Carlo treatment delivery model was developed for the Dynamic Collimation System (DCS) to evaluate the sensitivity of a simulated asynchronous delivery between the beam scanning controller and the preprogrammed sequencing DCS trimmer blades. Treatment deliveries were simulated in Geant4, which were modeled using a time series of the beam delivery system and DCS controller that discretizes a composite treatment simulation into fixed time points. Simulation executables were run in a Geant4 model of the DCS, incorporating a sigmoid kinematic model of DCS motion to derive simulation time points from log files obtained from patient-specific quality assurance measurements. Asynchronous deliveries were simulated by systematically offsetting the time series between the beam scanning controller and DCS and evaluating the constancy of the resultant dose distribution using gamma analysis.
Results
Communication delays between the PBS scanning controller and DCS account for 23% of the total treatment delivery time for collimated treatments, improving the treatment efficiency by nearly 22 seconds for each field if omitted. A continuous delivery approach that omitted communication delays exhibited moderated sensitivity to synchronization errors; the average 3%/3mm gamma pass rate for treatment fields simulated with a 100 ms offset was 94.8% (+/-0.5%).
Conclusion
DCS treatment efficiency may be further improved with a sliding-window delivery technique. However, monitoring the synchronization of these treatments would be necessary to ensure their delivered dosimetric accuracy.