Room: AAPM ePoster Library
Purpose: For non-radiographic localization systems that can be used for real-time tracking, the report of AAPM task group 147 recommends evaluation of the system latency. However, there are not well established techniques for measurement of latency. This work describes a simple measurement technique that requires only a motion phantom and an MV imager.
Methods: A wheeled platform driven by a programmable stepper motor was used to test three gating systems: RPM (Varian Medical Systems, Palo Alto CA), Calypso (Varian Medical Systems), and AlignRT (VisionRT, London, UK). A 6.35 mm stainless steel ball (BB) embedded in a foam block was placed on the motion platform and placed at isocenter by radiographic imaging. A system-specific target to trigger the gating system, such as a set of Calypso transponders, was also placed on the motion platform, which was programmed to move longitudinally between two positions symmetric about isocenter at speeds 1 to 2.6 cm/s. MV EPID images were obtained using integration mode and triggered via gating systems to visualize BB location. Without latency, the BB will be located at isocenter during irradiation; however, for a system with significant latency the BB will have moved away from isocenter when the MV radiation is triggered. The direction of movement will be dependent on which direction the BB was moving. Consequently, there will be two images of the BB symmetric about isocenter having separation proportional to the latency.
Results: The measured latency was < 100 ms for RPM and 173 ms for Calypso. For AlignRT at 4 frames per second, the latency was 476 ms, and was 825 ms at 2 frames per second.
Conclusion: The measured latencies were consistent with expected values. The reported technique is simple and uses equipment that should be readily available to clinics that use non-radiographic systems for gating.