Author: Hiroki Katayama, Hiroki Kawasaki, Motonori Kitaoka, Yosuke Takahashi, Takashi Tanii, Masato Tsuzuki 👨🔬
Affiliation: Kagawa University Hospital 🌍
Purpose:
Surface-guided radiotherapy (SGRT) with ExacTrac Dynamic (ETD) allows motion tracking and beam gating during treatment delivery. Accurate evaluation of beam-on and beam-off delays is essential for SGRT-based gating systems. This study assessed the time delays of a SGRT-based gating system using ETD in combination with the TrueBeam linear accelerator.
Methods:
A phantom with an embedded metal ball was placed on a motion platform, with the ball positioned at the isocenter as a reference. The phantom’s surface was assigned as the surface tracking area (STA), and a gating window of ±5 mm was set around the isocenter. Gated irradiation was performed while moving the phantom in the longitudinal direction. Beam delivery and electronic portal imaging device (EPID) imaging were triggered when motion within ±5 mm was detected. The position of metal ball on EPID images was analyzed to measure beam-on and beam-off delays. The relationship between these delay times and STA size as well as dose rate was investigated.
Results:
At a dose rate of 600 MU/min with 6XFFF, the beam-on delays were 314 ms ± 56 ms for the large STA shape and 308 ms ± 45 ms for the small STA shape (p = 0.297). The respective beam-off delays were 194 ms ± 36 ms and 189 ms ± 49 ms, respectively (p = 0.478). Beam-off delays were consistently shorter than beam-on delays. At 1200 MU/min, the beam-on and beam-off delays were 308 ms ± 33 ms and 213 ms ± 36 ms, with no significant differences compared to the delays at 600 MU/min (p = 0.122 and p = 0.571, respectively)
Conclusion:
Beam-on and beam-off delays did not depend on STA size or dose rate. Additionally, beam-off delays were consistently shorter than beam-on delays. These findings are expected to help optimize system configurations to improve gating accuracy.