ESTRO 38 Abstract book

S211 ESTRO 38

compensation (OOPM) 2 , Fayad model (FAY) 3 , ROI-based model (ROI) 4 and Stemkens model (STEM) 5 . Comparison among these methods was performed by means of a digital phantom. 3D Digital XCAT-based MRI phantoms were generated from six patient-measured tumour shapes, positions and motion signals: 4DMRI (mean cycle) was generated prior to treatment, with sagittal/coronal 2Dcine-MRI acquired during treatment (1x1x3mm 3 resolution, 300ms acquisition). Quantitative analysis was performed by comparing the estimated 3D volume to the ground truth available for each 2Dcine-MRI using the centre of mass distances (CD) and dice coefficient (DSC) for the tumour and the apex distance for the diaphragm (Dd). Results Figure A shows the boxplot for each method over the six cases (Friedman test for statistical analysis). For the tumour region (patient motion of 0.4-6.3mm) better results were achieved by PROP and ROI with an overall median CD of 1.14mm and 1.44mm, respectively. Higher errors and variabilities were instead quantified for OOPM, FAY and STEM, with CD>2mm (DSC<0.90) for patients with irregular breathing patterns (CD<1.8mm and DSC>0.92 for PROP and ROI). All methods presented median errors on Dd below 2.5mm (patient motion of 2.2-9.3mm), with PROP and ROI being the most accurate (Dd<3.1mm for irregular patients). Figure B shows a qualitative result for a regular and irregular patient. Conclusion A comparison between different strategies to derive time- resolved 3DMRI is proposed, with PROP and ROI outperforming other methods due to the capability to directly compensate for in-room variations and to account for regional changes, respectively. Future analysis will evaluate the performance of each method according to relevant patient-specific characteristics.

1 Paganelli et al. 2018. Feasibility study on 3D image reconstruction from 2D orthogonal cine‐MRI for MRI‐guided radiotherapy. JMIRO 2 Seregni et al. 2017. Out-of-plane motion correction in orthogonal cine-MRI registration. Radiother Oncol 3 Fayad et al. 2012. A generic respiratory motion model based on 4D MRI imaging and 2D image navigators. IEEE 4 Garau et al. 2018. A ROI-based global motion model for MRI-guidance in radiation therapy: a phantom study. Radiother Oncol 5 Stemkens et al. 2016. Image-driven, model-based 3D abdominal motion estimation for MR-guided radiotherapy. PMB OC-0410 Soft-tissue based on-line prostate motion assessment in 4D Cine-MR for MR-Linac treatments D. De Muinck Keizer 1 , L.G.W. Kerkmeijer 1 , M. Maspero 1 , J.R.N. Van der Voort van Zyp 1 , C.A.T. Van den Berg 1 , B.W. Raaymakers 1 , J.J.W. Lagendijk 1 , H.C.J. De Boer 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands Purpose or Objective We develop real-time MR-guided extremely hypofractionated (HF) prostate radiotherapy (RT) with active correction for prostate intrafraction motion. We have collected an extensive 4D cine-MR dataset to study the intrafraction motion of the prostate during the period of a RT fraction. Previously, we have presented a method for accurate automatic prostate tracking based on fiducial gold markers (ESTRO 37). Now we present a method for soft-tissue contrast based tracking that obleviates the need for fiducial markers on an MR-Linac. Material and Methods Thirty patients undergoing HF prostate RT had repeated cine-MR imaging sessions after each of five weekly fractions in a multicenter Medical Ethics board approved study. Each cine-MR session consisted of 55 sequentially obtained 3D datasets (‘dynamics’), acquired with a balanced 3D gradient echo sequence and a voxel spacing of 0.96x0.96x2mm 3 . Each dynamic was acquired over an 11 second period, with the cine-MR session covering a 10 minute period, similar to the duration of a RT fraction.A clinician delineated the prostate on the first dynamic from