Room: Davidson Ballroom A
Purpose: Motion management system latency is often composed of data acquisition, data processing, and linac beam triggering latencies. While there is a certain degree of control in how quickly imaging data is acquired and processed, the physical constraints of the linac make the beam triggering speed largely fixed. Quantifying the fixed beam latency can provide an understanding of its relative impact on the overall system latency and should be accounted for when developing motion management systems.
Methods: The latency associated with beam-on/off triggering for a Varian Clinac 21EX was investigated. Gating was performed using custom hardware designed to enable the beam with an externally generated pulse. Measurements were performed using a multi-channel oscilloscope to compare the time at which a gate signal was sent to the linac to turn the beam on/off, and the time at which the signal was first observed by a diode positioned at the beam isocenter. The linac latency was measured for 6 MV and 18 MV photons, and for dose rates ranging between 100 MU/min and 600 MU/min. Ten measurements were taken for each energy and dose rate combination.
Results: The beam-on and beam-off latencies were observed to be less than 20 ms for all dose rates and energies measured, with latencies of less than 10 ms at dose rates greater than 400 MU/min. Latency values were also shown to be dependent on both energy and dose rate, with observed beam-on and worst-case beam-off latencies reducing with increasing dose rate and decreasing energy.
Conclusion: The linac triggering latency was determined to have minimal contribution relative to the total system latency typically associated with motion management systems. Thus, the greatest potential for overall latency reduction is in the controllable aspects of system development, including data acquisition and processing methods.
Funding Support, Disclosures, and Conflict of Interest: This work was partially funded by NIH grants R01CA190298 and T32CA009206
Linear Accelerator, Gating, Quality Assurance
TH- External beam- photons: Quality Assurance - Linear accelerator