ESTRO 2022 - Abstract Book

S13

Abstract book

ESTRO 2022

Conclusion MG4dOPD is an effective approach for preserving the advantage of 4D optimized plans for PBS proton therapy delivery to lung cancer, if motion variability could be restricted by e.g. on-line visual feedback.

[1] A. Duetschler et al. 2021 Radiother Oncol. 161:S260-S261 [2] Ye Zhang et al 2013 Phys. Med. Biol. 58 8621

OC-0040 Gating latencies and resulting geometrical errors at clinical proton and photon accelerators

E. Worm 1 , J.B. Thomsen 2 , J.G. Johansen 2 , P.R. Poulsen 2

1 Aarhus University Hospital, Oncology, Aarhus, Denmark; 2 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark Purpose or Objective In respiratory gated radiotherapy, a low latency between target entrance/exit of the gating window and actual beam on/off is crucial for high treatment accuracy. However, no standard method to determine the latency exists. We used an in-house developed method (Thomsen et al, ESTRO 2021) for accurate latency measurements at a clinical proton and photon accelerator and simulated the geometrical consequences for liver SBRT treatments. Materials and Methods Gating latencies were measured at a Varian ProBeam (protons, RPM gating system) and TrueBeam (photons, TrueBeam gating system) accelerator, respectively. A motion-stage performed 1cm, 1Hz vertical sinusoidal motion of a marker block that was optically tracked by the gating system. Amplitude gating levels were set to cover approx. half the motion. Gated beams were delivered to a 5mm cubic scintillating ZnSe:O crystal that emitted visible light when irradiated. During beam- delivery, a GoPro video camera acquired images at 120Hz of the moving marker block and light emitting crystal. After treatment, the marker block position (black dot, Fig.1A) and crystal light intensity were determined in all video frames (Fig.1A). The video was then synchronized with the gating system log files by least squares-based alignment of the marker block motion in the video and in the gating log files (Fig. 1B-C). The gating latencies were measured over 15 breathing cycles and defined as the time difference between marker entrance/exit of the gating window and actual beam on/off as detected by the crystal signal (Fig.1B-C). For TrueBeam, the dose rate (and thus crystal signal) increased/decreased over an extended period of ≈ 30ms at each beam on/off and the latencies were measured relative to the mid-time of this 30ms interval (Fig.1B).