ESTRO 38 Abstract book

S919 ESTRO 38

irradiation at the MR-Linac using a double spin-echo sequence. The gel was additionally scanned on a 3 T MR device (Biograph mMR, Siemens) 5 h and 52 h after irradiation using multi spin-echo sequence with 32 equidistant echoes to evaluate the influence of different scanners on the star shot evaluation (Mephisto, PTW). To identify geometric distortions of the clinically used MR sequence (steady-state coherent sequence) of the MR- Linac, the individual control points of the grid were compared to CT-measurements as ground truth. The automatic determination of the control points was performed using the Weka segmentation (Image J, NIH). Results The evaluation revealed an isocircle radius of 0.5 mm for both imaging devices, image resolutions (0.5 x 0.5 mm² and 1.0 x 1.0 mm²) and time points after irradiation. The distance of the irradiation to the imaging isocenter was 0.25/0.65 mm (0.5 mm resolution) and 1.3/1.03 mm (1.0 mm resolution) for MR-Linac /3 T MRI respectively. Measurements of the grids of the QA-phantom revealed distortions below 1.2 mm (mean over all control points 0.35 mm) up to a distance along the bore of 102 mm from the isocenter. Conclusion The evaluation of a star shot measurement in a 0.35 T MR- LINAC is feasible, even immediately after irradiation. No significant differences for isocircle radii and isocentre distances between a diagnostic 3T MR scanner and the MR- Linac machine were found. As the PG can also be evaluated in 3D, future experiments will focus on a 3D isocenter alignment test where several slices along the y- direction will be evaluated. This would allow identifying any misalignment of Linac-components. EP-1708 Organ motion impact on dose delivered with non-coplanar VMAT for lung SBRT A. Bazani 1 , S. Comi 2 , F. Pansini 1 , F. Emiro 1,3 , D. Ciardo 4 , G. Piperno 4 , A. Ferrari 4 , B.A. Jereczek-Fossa 4,5 , F. Cattani 1 , C. Garibaldi 6 1 IEO- European Institute of Oncology IRCCS, Unit of Medical Physics, Milan, Italy ; 2 European Istitute of Oncology, Radiotherapy, Divisione di Radioterapia pres, Italy ; 3 University of Milan, Physics, Milan, Italy ; 4 IEO- European Institute of Oncology IRCCS, Department of Radiation Oncology, Milan, Italy ; 5 University of Milan, Department of Oncology and Hemato-Oncology, Milan, Italy ; 6 IEO- European Institute of Oncology IRCCS, Radiation Research Unit, Milan, Italy Purpose or Objective To evaluate the effect of organ motion on dose delivered with non-coplanar VMAT for lung stereotactic treatment. Material and Methods Dynamic Wave Arc (DWA) is a novel non-coplanar VMAT technique implemented on the VERO SBRT system. The fluence modulation is achieved by a synchronized moving of gantry, ring and leaves at a fixed dose rate (400 MU/min). Seven DWA highly modulated VMAT treatments were planned for a single lung patient using Raystation TPS (v7.0, 0.2 mm dose grid) with Collapsed Cone Convolution Algorithm (v3.5) on a 4DCT scan(2.5 mm slice width). Plans were optimized on the mean CT obtained from 10 breathing phases. The goal prescription was D 95% =95% for ITV+5mm. So far, 3 arc templates, simulating different combinations of gantry/ring speeds, 2 dose levels (15Gyx3 fr and 7.5Gyx8 fr) and 2 extreme organ motion amplitudes (2 and 4 cm)were considered to assess the impact of organ motion on dose delivery. Two ITVs were created from the original GTV to simulate a peak-to-peak tumor motion of 2 and 4 cm in cranio-caudal direction. Plans were delivered on the Delta4 with HexaMotion 6D Motion Platform. Dose distributions obtained with the phantom moving according to different 1D sinusoidal motion patterns (A = 2, 4 cm, T = 2, 4, 6 s, 3 phase shifts with respect to the start of the treatment) were compared with

The purpose of this research is to develop modified materials for 3D printing with specified CT indices. Material and Methods Fused deposition modeling was chosen for the production of customized phantoms due to the possibility of simultaneously producing objects from several materials. To estimate the values of CT indices, test samples with a variety of infill values were manufactured on an UP! Plus 2 3D printer from pure polylactide (PLA) filaments and modified filaments. The 2-cm 3 samples were printed with a diamond infill pattern. The modified filaments were made using a single-screw extruder SJ 45/25 with a diameter of 45 mm and an L/D ratio of 25 from a mixture of PLA and copper powder (particle size 10-20 μm). The CT indices were measured using a Siemens Somatom Emotion computed tomography scanner. The data obtained were processed by the eFilm software. Results In the eFilm software, the tomograms were presented in the form of high-contrast black and white images. Each shade of gray was assigned a numeric value that corresponded to a specific value on a Hounsfield unit scale (CT indices). The samples were positioned parallel and perpendicular to the rotation axis for CT scanning. Based on the processed data, a linear dependence of the CT indices on the sample infill values was obtained. It is shown that CT indices depend on the sample orientation in a CT scanner due to the different number of air gaps lying in the CT scanning plane. Conclusion The obtained results show that pure PLA can be used to simulate soft tissues with densities ranging from -600 to +200 on the Hounsfield scale. Metal-modified materials allow us to simulate muscle and bone tissue from +100 to -1100 HU. The obtained dependences can be used to accurately determine the required material infill value for obtaining a specific CT index. That will allow applying the developed materials to produce customized tissue-equivalent phantoms. EP-1707 Polymer gel-based tests for geometric accuracy in a 0.35 T MR-LINAC S. Dorsch 1,2 , P. Mann 1 , A. Elter 1,2 , A. Runz 1 , S. Klüter 3 , C.P. Karger 1 1 German Cancer Research Center DKFZ, Medical Physics in Radiation Oncology, Heidelberg, Germany ; 2 University of Heidelberg, Faculty of Physics and Astronomy, Heidelberg, Germany ; 3 University Hospital Heidelberg, Department of Radiation Oncology, Heidelberg, Germany Purpose or Objective As MRgRT is becoming increasingly important in clinical applications, the development of new QA methods is needed. Besides the QA for dose delivery and dosimetry, also the machine-related geometric parameters like the alignment of the irradiation and imagining isocenters as well as the detection and measurement of geometric distortions of the MR are of key importance. Polymer dosimetry gels (PG) may offer a way to perform isocenter alignment tests as PG is visible in MR imaging. Additionally, PG may allow for online 3-dimensional (3D) evaluation. For this purpose we present first measurements of a dedicated phantom to measure both, isocenter alignment with a PG and geometric distortions with a regular 3-dimensional grid. Material and Methods The in-house developed phantom contains a PG (PAGAT 1 )- filled spherical glass flask embedded in 8 grid sheets building up a 3D grid of 996 control points. To measure the alignment of the irradiation and imaging isocenter, the phantom was irradiated with 5 beams separated by equidistant angles at a clinical MR-Linac (MRIdian, ViewRay) and was scanned immediately and 48 h after