A simple method to measure the gating latencies in photon and proton based radiotherapy using a scintillating crystal

Abstract

Background: In respiratory gated radiotherapy, low latency between target motion into and out of the gating window and actual beam-on and beamoff is crucial for the treatment accuracy. However, there is presently a lack of guidelines and accurate methods for gating latency measurements. Purpose: To develop a simple and reliable method for gating latency measurements that work across different radiotherapy platforms. Methods: Gating latencies were measured at a Varian ProBeam (protons, RPM gating system) and TrueBeam (photons, TrueBeam gating system) accelerator. A motion-stage performed 1 cm vertical sinusoidal motion of a marker block that was optically tracked by the gating system. An amplitude gating window was set to cover the posterior half of the motion (0-0.5 cm). Gated beams were delivered to a 5 mm cubic scintillating ZnSe:O crystal that emitted visible light when irradiated, thereby directly showing when the beam was on. During gated beam delivery, a video camera acquired images at 120 Hz of the moving marker block and light-emitting crystal. After treatment, the block position and crystal light intensity were determined in all video frames. Two methods were used to determine the gate-on (τ on ) and gate-off (τ off ) latencies. By method 1, the video was synchronized with gating log files by temporal alignment of the same block motion recorded in both the video and the log files. τ on was defined as the time from the block entered the gating window (from gating log files) to the actual beam-on as detected by the crystal light. Similarly, τ off was the time from the block exited the gating window to beam-off. By method 2, τ on and τ off were found from the videos alone using motion of different sine periods (1-10 s). In each video, a sinusoidal fit of the block motion provided the times T min of the lowest block position. The mid-time, T mid-light , of each beam-on period was determined as the time halfway between crystal light signal start and end. It can be shown that the directly measurable quantity T mid-light − T min = (τ off +τ on )/2, which provided the sum (τ off +τ on ) of the two latencies. It can also be shown that the beam-on (i.e., crystal light) duration ΔT light increases linearly with the sine period and depends on τ off − τ on : ΔT light = constant•period+(τ off − τ on ). Hence, a linear fit of ΔT light as a function of the period provided the difference of the two latencies. From the sum (τ off +τ on ) and difference (τ off − τ on ), the individual latencies were determined. Results: Method 1 resulted in mean (±SD) latencies of τ on = 255 ± 33 ms, τ off = 82 ± 15 ms for the ProBeam and τ on = 84 ± 13 ms,τ off = 44 ± 11 ms for the This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

Comprehensive beam delivery latency evaluation for gated proton therapy system using customized multi‐channel signal acquisition platform

“…investigated the proton beam delivery latency within the same gated proton treatment system (ProBeam + RPM version 1.75) using a scintillating crystal and a consumer‐grade camera. 14 Their reported average latencies of τ total‐on = 255 ± 23 ms and τ total‐off = 95 ± 23 ms agree with our measured average latencies of τ total‐on = 212.50 ± 13.64 ms and τ total‐off = 114.80 ± 14.98 ms. Our τ total‐on was approximately 40 ms smaller, which could be caused by RPM system upgrade (version 1.8 vs. version 1.75). As noted by Worm et al., differences in the beam ON/OFF switch mechanisms of the cyclotron result in a discrepancy between τ total‐on and τ total‐off .…”

“…The proton beam can be rapidly dumped using deflector plates, but it takes a relatively longer time to be re‐established. 14 This conclusion was quantified by our cyclotron system gating delay results of τ CYC‐off = 1.20 ± 0.04 ms and τ CYC‐on = 108.29 ± 0.85 ms. According to the best of our knowledge, the cyclotron system gating latencies and their contributions to the total beam delivery latencies during gated proton treatments have not been previously reported in the published literature.…”

“…Simultaneously, a camera operating at a 120 Hz sampling rate captured simulated vertical motions. 14 The only disadvantage was its focus on total gating latency measurements, without the ability to separate the delays within the motion management system and the beam delivery system from the end‐to‐end latencies. In terms of measurement accuracy and compatibility, the method proposed by Wiersma et al.…”

“…Similar to the approach by Worm et al., independent gating delay measurements for both the respiratory gating system and treatment system were not feasible. 14 …”

“…Therefore, some specialists in the area of medical physics have developed individual solutions to measure the gating latencies. 10 , 11 , 12 , 13 , 14 , 15 All these solutions only focused on the end‐to‐end latency measurements with various gated treatment systems. The objective of this work was to design a reliable multi‐channel signal acquisition platform to measure the respiratory gating system delays and the proton treatment system delays separately under different simulated breathing patterns.…”