ESTRO 2020 Abstract book

S172 ESTRO 2020

at planning 4D-CT and pretreatment 4D-CBCT for lung SBRT patients in order to validate whether the respiration induced motion during simulation is representative during treatment. Material and Methods In total 461 patients were treated with periphery lung SBRT at a single institution in a period of two years from September 2017. For 7 patients no proper 4D CT could be obtained, and further 7 patients had no 4D CBCT (typically due to GTV volumes <1 cc). Of the remaining patients, 35 had 2 tumors and 4 had 3 tumors. Thus, motions of 490 tumors in 447 patients were analyzed. The 4D CT scan was analyzed in MIM to determine peak-to-peak motion using a method involving deformable propagation of the delineated target to all phases. 4D CBCT data was obtained and analyzed by XVI 5.0 on Elekta linacs and extracted. The peak-to-peak motion was thereby determined at the time of simulation and first treatment fraction. Patient by patient, the peak-to-peak motion at simulation and peak- to-peak motion at treatment was coupled, and differences were analyzed. Results At simulation, a total of 82.0% of the patients had a peak- to-peak motion <1 cm and 96.3% had <2cm. A scatter plot of peak-of-peak motion is show in figure 1 together with a line of identity (y=x) for the CC direction for all patients. Most of the patients with very different motion (far away from y=x) in figure 1 was evaluated individually and the main reason to the difference was linked to poor image quality. Figure 2 shows the data as a Bland-Altman plot. Mean and standard deviation for all the patients in the CC direction was -0.03 and 0.33 cm, respectively. For patients with a peak-to-peak motion <1cm at simulation the similar values are -0.06 and 0.26 cm (-0.05 ± 0.29 cm for <2cm).

Conclusion We present a tool that allows efficient evaluation of breathing motion uncertainties using a combined ROM-NN approach. When facing breathing interplay, both plans perform similarly, with ITV plans marginally outperforming 4D-CT approach. However, a much lower computational cost (1/20) may tip the balance in favour of ITV robust optimization.

Poster discussion: PH: Adaptive radiotherapy and inter- fraction motion management 1

PD-0307 Tumor motion at planning 4D-CT and treatment 4D-CBCT for a large cohort of lung SBRT targets T.B. Nielsen 1 , C. Brink 1,2 , S.S. Jeppesen 3 , T. Schytte 2,3 , O. Hansen 2,3 , M. Nielsen 1 1 Odense University Hospital, Laboratory of Radiation Physics, Odense, Denmark ; 2 Faculty of Health Science- University of Southern Denmark, Department of clinical research, Odense, Denmark ; 3 Odense University Hospital, Department of Oncology, Odense, Denmark Purpose or Objective Treatment margin of SBRT of lung cancer patients will depend on the size of the respiration induced motion. The aim of this study is to evaluate and compare tumor motion

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