ESTRO 2023 - Abstract Book

S1638

Digital Posters

ESTRO 2023

Table 1 Difference in surface tracking compared to X-ray image verification before and after intentional shifts.

Conclusion The results demonstrated that surface tracking could differ by up to 0.5 mm depending on skin tone, texture, and camera equilibrium state. Thermal imaging of real patients may reduce this difference. X-ray image verification should be rigorously used with surface tracking for dark moist-skinned patients.

PO-1893 Gated dynamic trajectory radiotherapy: feasibility, mechanical accuracy and dosimetric validation

H.A. Loebner 1 , D. Frauchiger 1 , D. Terribilini 1 , S. Mueller 1 , G. Guyer 1 , P. Mackeprang 1 , M.F.M. Stampanoni 2 , M.K. Fix 1 , P. Manser 1 , J. Bertholet 1 1 Inselspital, Bern University Hospital and University of Bern, Division of Medical Radiation Physics and Department of Radiation Oncology, Bern, Switzerland; 2 ETH Zuerich and PSI, Institute for Biomedical Engineering, Villigen, Switzerland Purpose or Objective By extending VMAT with dynamic table and collimator rotation during beam-on, dynamic trajectory radiotherapy (DTRT) has shown improved organ at risk sparing compared to VMAT. To deliver the intended dose distribution to a moving anatomy, motion mitigation, e.g. by gating, can be applied. However, this may be a challenge due to the increase in dynamic degrees of-freedom of DTRT. The aim of this work is to experimentally assess the technical feasibility, quantify the mechanical accuracy and validate the dosimetric performance of respiratory gating during DTRT delivery. Materials and Methods An HexaMotion motion stage with a Delta4 phantom positioned on the treatment table of a TrueBeam system is used to mimic four different breathing motion traces: typical, high frequency, predominantly left-right and baseline shift, each with combined motion in superior-inferior, anterior-posterior, and left-right directions. A DTRT plan for a clinically motivated lung case is delivered both with and without gating in developer mode on a TrueBeam system. For gating, the real-time positioning management (RPM) system is used to trigger amplitude gating based on the main motion axis (either superior-inferior or anterior-posterior) with gating window between 4 and 6 mm depending on the motion trace. Mechanical accuracy is assessed from TrueBeam trajectory log files as the deviation between actual and expected machine positions for gantry, table and collimator angle and positions of moving MLC leaves. Dosimetric accuracy is determined by comparing the calculated dose to the measured dose using the Delta4 phantom by means of the gamma analysis (3% global/2 mm, 10% threshold). Additionally, the dosimetric accuracy is assessed for a DTRT delivery and a static phantom. Results DTRT delivery was successfully gated with the gantry, table and collimator rotating back during beam-hold and resuming motion at beam-on (i.e., when entering the gating window). The deviation between expected and actual machine position for deliveries with and without gating are shown in figure 1. The spread of the mechanical deviations is slightly smaller for deliveries with gating, which is believed to be related to the reset of lagging machine components at gating events. Deliveries with gating (resp. without) achieved gamma passing rates >96% (<88%) (figure 1). For the static phantom, the passing rate was 99%.