ESTRO 38 Abstract book

S1075 ESTRO 38

treatment. Future experiments will focus on the verification of dose accumulation algorithms that are used to calculate the final dose in case of dynamic treatment. EP-1971 Comparison of pancreatic respiratory motion using three abdominal corsets for particle therapy S. Schneider 1,2 , K. Dolde 3,4,5 , M. Alimusay 6 , B. Fluegel 6 , N. Saito 7 , A. Hoffmann 1,2,8 , A. Pfaffenberger 3,4 1 Helmholtz-Zentrum Dresden-Rossendorf, Radiooncology- OncoRay, Dresden, Germany ; 2 OncoRay - National Center for Radiation Research in Oncology, Medical Radiation Physics, Dresden, Germany ; 3 German Cancer Research Center DKFZ, Medical Physics in Radiation Oncology, Heidelberg, Germany ; 4 National Center for Radiation Research in Oncology, Heidelberg Institute for Radiooncology, Heidelberg, Germany ; 5 University of Heidelberg, Department of Physics and Astronomy, Heidelberg, Germany ; 6 Heidelberg University Hospital, Center for Orthopedic and Trauma Surgery, Heidelberg, Germany ; 7 University Clinic Heidelberg, Department of Radiation Oncology, Heidelberg, Germany ; 8 Faculty of Medicine and University Hospital Carl Gustav Carus at the Technische Universität Dresden, Radiotherapy and Radiation Oncology, Dresden, Germany Purpose or Objective Particle therapy (PT) has the potential to reduce the risk of toxicity in patients with pancreatic cancer beyond that of photon therapy. Since PT is strongly susceptible to anatomical changes during dose delivery, strategies for pancreatic motion management are mandatory. The purpose of this study was to compare the usability of different corsets for abdominal compression by measuring their effect on the respiration-induced motion of the pancreas as well as their water equivalent ratio (WER) for PT. Material and Methods Three types of abdominal corsets were used that differed in material composition (polyethylene (PE) vs . polyurethane (PU)), material thickness (2.5mm – 24mm), and construction method (patient individual vs . patient independent). A healthy volunteer was scanned on a 1.5T MR scanner (Magnetom Aera, Siemens Healthineers) on two consecutive days for four subsequent scenarios: with and without wearing the three corsets. A gradient echo sequence with radial golden angle acquisition (field of view = 384×384×288 mm 2 , voxel size = 1.5×1.5×3 mm 3 , spokes per partition = 2100, bandwidth = 610 Hz/pixel, TE = 1.5 ms, TR = 3.3 ms, ) was used and reconstructed to a 4D-MR data set comprising 20 breathing phases. The pancreas was delineated in the maximum inhale and exhale phases using the open-source software MITK (Fig. 1). Its centre of mass was calculated as a surrogate for the respiratory motion in each of the four scenarios for both days. CT scans of the three corsets were acquired (field of view = 388×388×476mm³, voxel size = 0.73×0.73×2mm³, tube voltage = 120kVp, Tube current time product = 148mAs) to assess the material homogeneity and thickness constancy. WER measurements were performed at two different proton energies (150MeV, 200MeV) using a high-resolution multi-layer ionization chamber (Giraffe, IBA Dosimetry).

EP-1970 2D and 3D dose verification for a gated irradiation on a 0.35 T MR-LINAC P. Mann 1,2 , K. Spindeldreier 2,3 , G. Echner 1,2 , S. Klüter 2,3 , C. Karger 1,2 1 German Cancer Research Center DKFZ, Medical Physics in Radiation Oncology, Heidelberg, Germany ; 2 National Center for Radiation Research in Oncology NCRO, Heidelberg Institute for Radiation Oncology HIRO, Heidelberg, Germany ; 3 University Hospital Heidelberg, Department of Radiation Oncology, Heidelberg, Germany Purpose or Objective As MRgRT becomes increasingly important in clinical applications, the development of new QA methods is needed. Especially for the verification of MR-based gating techniques, workflows tests with 2D and 3D dose verification prior clinical implementation are desirable. In this work, we present an MR compatible motion phantom that can hold both film and polymer gel (PG) inserts to perform 2D and 3D dose measurements for gated treatments. Material and Methods A water-filled cylindrical phantom (d = 22 cm, l= 50 cm) holding a movable smaller water-filled cylinder (d = 9 cm, l = 50 cm) that can be combined with (1) film and (2) PG inserts was developed. For (1), an elliptical silicone insert with high MR contrast served as a target, while for (2), an oxygen sealed BAREX TM container filled with in-house produced PAGAT dosimetry gel was used as target. Both inserts were irradiated in separate experiments at an MR- Linac (MRIdian, ViewRay © ) with a dose of up to 5 Gy. To test the influence of motion and gating on the target coverage with dose, three different irradiations were carried out: (i) Static target (ii) Dynamic target without gating (iii) Dynamic target with gating For all experiments, the target volume was delineated in the Viewray TPS. Phantom motion for (ii) and (iii) was set to a cos 4 trajectory with an amplitude of 1.5 cm and a motion cycle of 10 cycles/min. For (iii) an additional gating margin of 3 mm was added allowing for 10% of the target being outside of the contour. The film was scanned (10000XL, Epson) 1 h after irradiation and the red channel was evaluated . PG was scanned on a 3 T MR device (Biograph mMR, Siemens) 48 h after irradiation using a multi spin-echo sequence. Results were evaluated using the 2/3D -criterion (2%/2mm and 3%/3mm). For the film, the relative profiles were compared (using experiment (i) as reference) and the PG results were compared against TPS calculations. Results For both film and PG measurements, results showed a homogeneous target coverage for (i), a significant dose smearing in motion direction for (ii) and a restored dose distribution with a homogeneous target coverage for (iii). The respective passing rates are shown in table 1.

Conclusion The developed phantom in combination with film and PG inserts has shown to be a valuable tool to test new treatment techniques like gating on a commercial MR- Linac device. Three different scenarios have been tested and gating revealed nearly the same accuracy as a static