ESTRO 37 Abstract book

S1213

ESTRO 37

phantom with known motion and measuring the actual movement on images. Material and Methods Various motion waveforms were applied to a 3.3 cm diameter sphere using the Quasar MRI4D motion phantom (ModusQA, London, ON, Canada). Respiratory cycles were configured with different sinusoidal waves with 4, 5 and 6 s period and 10, 20 and 30 mm peak to peak amplitude. The phantom was firstly scanned in a Magnetom Aera 1.5 T (Siemens Medical Solutions, Erlangen, Germany) using an 18-channel design body flex coil. The 4D MRI sequence consists of an interleaved acquisition of 2D data slices and navigators. The navigator position is fixed in sagittal orientation to extract the respiratory pattern of the phantom while the data slices are acquired in axial orientation and cover the volume of interest 3 .Data slices are then binned according to the respiratory information. For comparison with a conventional motion management technique in radiotherapy, the same phantom was imaged on a Optima RT32 4D CT scan (GEMS, Waukesha, WI). Quantitative evaluation consisted of comparing against the actual values the sphere volume on each phase of the 4D image sets as well as the peak to peak motion amplitude. Results The sphere volume is generally overestimated on MRI (up to 29%). For a given wave amplitude of 20 mm, the sphere volume measured on MRI increases with breathing period (from 7 to 29%). A similar effect was observed on CT images. For a given breathing period of 6s, the sphere volume measured on MRI increases with wave amplitude. This effect is less pronounced on CT images. The sphere displacement is underestimated with both modalities (up to 20%). For a given wave amplitude of 20 mm, the displacement amplitude was better estimated on MRI for the smallest period of 4s whereas CT gave the worst estimate for this period. For a given breathing period of 6s, the displacement amplitude was underestimated on MRI by 20 % whatever the displacement amplitude, while on CT the worst estimate was obtained for the largest displacement. Conclusion This phantom study showed that the new 4D MRI sequence succeeded in imaging a moving target with similar performances as a classical 4D CT. The overestimation of sphere volume on MR images is not specific of the 4D sequence as it was also observed on static images (20% overestimation). These promising results encouraged us to pursue this work with clinical applications. 1. M.von Siebenthal et al, Phy.Med.Biol. 2007 2. Z.Celicanin et al, Magn Reson.Med. 2015 3. S.Ken et al, Magnetom 2017 EP-2191 Design of a national survey to assess the technology applied to SBRT C. Marino 1 , C. Carbonini 1 , I. Veronese 1 , S. Clemente 1 , M. Esposito 1 , C. Fiandra 1 , M. Fusella 1 , C. Garibaldi 1 , F. Giglioli 1 , E. Moretti 1 , S. Russo 1 , A. Savini 1 , L. Strigari 1 , S. Strolin 1 , C. Talamonti 1 , E. Villaggi 1 , M. Stasi 1 , P. Mancosu 1 1 SBRT Working Group, AIFM - Italian Association of Medical Physics, Italy, Italy Purpose or Objective Stereotactic body radiation therapy (SBRT) approach for treating early stage solid tumor and metastases is increasing worldwide. In 2013 the Italian Association of Medical Physicists (AIFM) constituted a working group in order to standardize SBRT dosimetric aspects. A survey was designed aiming to delineate the status of the technology applied in SBRT on a national level. Clinical evaluation of SBRT was out of the present study.

These results suggest ±2% from baseline as an action level for dose output, which was surpassed in the months leading up to an x-ray tube replacement, establishing a new baseline. For the imaging vs. radiation isocenter results the Z- and Y-direction offsets were derived from the 90° measurements, whereas the X-direction offsets were derived from 0° based on a lower resolution detector, explaining the larger variation in the X-offsets. We found an offset of 1mm to be an appropriate action level, requiring a re-calibration of the CBCT imaging system. Importantly, a 1mm offset was not easily detected visually from the images, but has considerable implications for the desired dose distribution with typical small animal irradiation field sizes. Conclusion Based on the long-term results presented here we were able to determine critical action levels that should be adhered to when assessing the performance of robotic small animal irradiators. This will provide investigators with confidence in the radiation delivery of their experiments and ensure the translational value of the results. EP-2190 Quantitative evaluation of a new 4D MRI sequence: a motion phantom study L. Parent 1 , T. Nemtanu 1 , Z. Celicanin 2 , O. Bieri 2 , P. Cattin 3 , S. Ken 1 1 Institut Universitaire du Cancer Toulouse - Oncopole, Engineering and medical physics, Toulouse, France 2 University of Basel Hospital, Radiology, Basel, Switzerland 3 University of Basel, Medical image analysis center, Basel, Switzerland Purpose or Objective A novel retrospective gating method for dynamic 3D MR imaging during free breathing was developed 1 and further improved by Celicanin et al 2 . This study aims at assessing quantitatively this new sequence by imaging a moving