ESTRO 2021 Abstract Book

S1478

ESTRO 2021

Conclusion Intrafractional motion detection and patient repositioning using X-ray imaging during spine SBRT can lead to improved safety during the application of high BED in critical locations for patients without additional immobilization. Further and more detailed analysis is necessary and may lead to improved clinical decision making to identify patients who benefit from further body immobilization during spine SBRT. PO-1753 The influence of beam and organ motion on pancreas proton irradiations B. Knäusl 1 , F. Lebbink 1,2 , P. Fossati 3 , E. Engwall 4 , T. Madar 2 , A. Carlino 2 , D. Georg 1 , M. Stock 5 1 Medical University of Vienna, Department of Radiation Oncology, Vienna, Austria; 2 MedAustron Center for Ion Beam Therapy and Research, Medical, Wiener Neustadt, Austria; 3 MedAustron Center for Ion Beam Therapy and Research, Medical , Wiener Neustadt, Austria; 4 RaySearch Laboratories, Physics, Stockholm, Sweden; 5 MedAustron Center for Ion Beam Therapy and Reserach, Medical, Wiener Neustadt, Austria Purpose or Objective The necessity for motion compensation in particle therapy depends on the anatomy, motion amplitude and underlying beam delivery technology. Although pancreas patients are not classified as large movers, there is room for improvement of existing treatment concepts. In this retrospective study the potential benefit of rescanning for pancreas proton treatment and the need for motion mitigation was investigated. Furthermore, time resolved dose prediction for clinical use was developed in this context. Materials and Methods The dose distributions of 13 pancreas patients treated with curative hypofractionated proton therapy were analysed based on accelerator logfiles, 4D computed tomography (CT) data and breathing patterns extracted from the 4DCT or, if available, from the surface scanner signal (Catalyst, C-Rad). Proton beam therapy was based on pulsed scanned pencil beams delivered by a synchrotron. Dose prescription was 25 and 37.5 Gy biologically weighted dose to the PTV1 (ITV of GTV and neuroplexuses and lymphnode station + 5mm) and PTV2 (high risk GTV + 5mm), respectively, in 5 fractions with a simultaneously integrated boost. If necessary,