SU‐E‐J‐126: Respiratory Gating Quality Assurance: A Simple Method to Achieve Millisecond Temporal Resolution

Abstract

Purpose: Low temporal latency between a gating on/off signal and a linac beam on/off during respiratory gating is critical for patient safety. Although, a measurement of temporal lag is recommended by AAPM Task Group 142 for commissioning and annual quality assurance, there currently exists no published method. Here we describe a simple, inexpensive, and reliable method to precisely measure gating lag at millisecond resolutions. Methods: A Varian Real‐time Position Management™ (RPM) gating simulator with rotating disk was modified with a resistive flex sensor (Spectra Symbol) attached to the gating box platform. A photon diode was placed at machine isocenter. Output signals of the flex sensor and diode were monitored with a multichannel oscilloscope (Tektronix™ DPO3014). Qualitative inspection of the gating window/beam on synchronicity were made by setting the linac to beam on/off at end‐expiration, and the oscilloscope's temporal window to 100 ms to visually examine if the on/off timing was within the recommended 100‐ms tolerance. Quantitative measurements were made by saving the signal traces and analyzing in MatLab™. The on and off of the beam signal were located and compared to the expected gating window (e.g. 40% to 60%). Four gating cycles were measured and compared. Results: On a Varian TrueBeam™ STx linac with RPM gating software, the average difference in synchronicity at beam on and off for four cycles was 14 ms (3 to 30 ms) and 11 ms (2 to 32 ms), respectively. For a Varian Clinac™ 21EX the average difference at beam on and off was 127 ms (122 to 133 ms) and 46 ms (42 to 49 ms), respectively. The uncertainty in the synchrony difference was estimated at ±6 ms. Conclusion: This new gating QA method is easy to implement and allows for fast qualitative inspection and quantitative measurements for commissioning and TG‐142 annual QA measurements.

Technical Report: TG‐142 compliant and comprehensive quality assurance tests for respiratory gating

“…The IR camera has an average delay of 17 (0-33) ms, as mentioned above. MV beam-on latency is the time between a gating trigger and actual beam-on, which was measured directly using a multichannel oscilloscope by McCabe and Wiersma, and reported to be between 3 and 30 ms. 18 showing the kV beam-on latency after trigger. The kV exposure time is 10 ms in our study and its variation can be obtained also from the TrueBeam specification: 10% for <5 ms and 2% for <10 ms.…”

“…Our results suggested a much higher value and can be confirmed by using the same method as described by McCabe and Wiersma. 18 Each of the above mentioned components has their inherent uncertainties, which resulted in a broad range of possible beam-on delays for both MV and kV beams. In addition, several extrinsic uncertainties exist, mostly from the imager special resolution, motion platform, and image measurement.…”

“…17 In a recent conference abstract, McCabe and Wiersma described their method of using a resistive flex sensor and diode monitored with a multichannel oscilloscope, to directly determine the gating window/beam-on synchronicity for Varian RPM and TrueBeam machines. 18 These beam on/off time delays vary with machines and x-ray beams; thus, it is important to establish a baseline value in time delay before using the gating feature, as suggested by TG-142.…”