ESTRO 2021 Abstract Book

S1677

ESTRO 2021

Results In 6 patients, organ motion was considered negligible (GTV=ITV). For treatment planning, the overall mean GTV and ITV were 7.5±7.3cc and 10.5±7.5cc, respectively. A total of 115 4D-CBCTs were acquired and evaluated. ITV of one patient was showed in figure 2. During treatment, the mean registered set-up error was -0.04 ± 0.25 mm in lateral direction, -0.04 ± 0.31 mm in longitudinal direction and 0.35 ± 0.30 mm in vertical direction. For all patients, GTV was always included in PTV.

Conclusion Our study confirmed Symmetry 4D-CBCT as a valid instrument for set-up and organ motion evaluation, reporting maximum shift deviation in vertical direction of 0.35 mm. Moreover, Symmetry resulted able to generate ITV replacing a 4D-CT simulation, as demonstrated by the inclusion of all GTVs in the ITVs. These favorable results could suggest to use the Symmetry 4D-CBCT as a useful methods for planning and delivery of thoracic stereotactic radiotherapy. PO-1971 Patient experience and gating efficiency using a visual tracking system for breath hold during MRgRT N. Weitkamp 1 , S. Ehrbar 1 , M. Chamberlain 1 , S. Perryck 1 , T. Rudolf 1 , L. Wilke 1 , H.I. Garcia Schüler 1 , S. Tanadini-Lang 1 , M. Guckenberger 1 , N. Andratschke 1 1 University Hospital Zürich, Radio-Oncology, Zürich, Switzerland Purpose or Objective Patients treated in breath hold on the MRIdian (Viewray®) have so far been depending solely on verbal commands from the RTT`s. The aim of this study was to implement a visual tracking device for patients and to evaluate the impact of visualizing the target volume on patient experience and gating efficiency.Patients treated in breath hold on the MRIdian (Viewray®) have so far been depending solely on verbal commands from the RTT`s. The aim of this study was to implement a visual tracking device for patients and to evaluate the impact of visualizing the target volume on patient experience and