ESTRO 36 Abstract Book

S943 ESTRO 36 2017 _______________________________________________________________________________________________

To study the performance, image quality and clinical potential of a novel fast MV-CBCT panel mounted on a straight-through linac with 6X-FFF (filter-free) beam. Material and Methods A prototype MV imager is mounted in-line with the linac at 154 cm source-to-imager distance. MV-CBCT imaging is performed with 28 cm fixed width (projected at isocenter- 100 cm) and variable field length (up to 26 cm max) FOV, 200º arc, 15 sec rotation time, at 5MU and 10MU dose levels. Images of a Catphan® 604 phantom are analyzed in terms of noise, uniformity, spatial and contrast resolution, and contrast linearity. The results are compare with those from Truebeam kV-CBCT imager, and potentially, a Tomotherapy MV-CT imager, all benchmarked to diagnostic CT scanner images. Imaging dose will also be assessed for completeness of testing of the MV-CBCT The uniformity and noise of the MV-CBCT was acceptable but not as good relative to diagnostic CT and kV-CBCT, with variation of 32HU from the center to the periphery. Spatial resolution is shown in Fig. 1 with 3lp/mm for 5MU and 4lp/mm for 10MU dose levels, as compared to 7lp/mm for both kV-CBCT and diagnostic CT. The low contrast resolution of the MV-CBCT was >> 1%, compared to 1.0% and 0.5% for kV-CBCT and diagnostic CT. Contrast linearity and sensitometry is shown in Fig. 2, with the MV-CBCT (and 10MU dose level) being the modality that deviates especially for higher density objects like Acrylic, Delrin and Teflon (green data points). Additional tests with varying field length (small, medium and max length) for the MV-CBCT did not show any correlation and/or improvement to the image quality results. Imaging dose will be assessed and presented based on absolute beam output calibration with a 6cc ion chamber and OSLDs. The Tomotherapy data and analysis is underway for comparisons. system. Results

Conclusion Preliminary image quality tests of a prototype MV-CBCT imaging system that utilizes a 6X-FFF beam and a fast acquisition mode revealed acceptable performance, but as expected, worse than those of kV-CBCT and diagnostic CT. The fast acquisition may potentially be beneficial for motion management treatments such as DIBH for breast and lung tumors. The MV-CBCT also has superior contrast linearity for higher density and metallic materials because of reduced beam hardening, and could be utilized to supplement diagnostic CT images for treatment planning in such cases. EP-1739 The feasibility of atlas-based automatic segmentation of MRI for prostate radiotherapy planning C. Morris 1 , M.J. Gooding 2 , A. Henry 3 , R. Speight 4 1 University of Leeds, Department of Medicine, Leeds, United Kingdom 2 Mirada Medical Ltd, Mirada Medical Ltd, Oxford, United Kingdom 3 St James Institute of Oncology, Department of Clinical Oncology, Leeds, United Kingdom 4 St James Institute of Oncology, Medical Physics and Engineering, Leeds, United Kingdom Purpose or Objective Atlas-based autosegmentation is an established tool for segmenting structures for CT-planned prostate radiotherapy. MRI is being increasingly integrated into the planning process. The aim of this study was to assess the