Abstract Book

S1166

ESTRO 37

EP-2119 Single vs. Multi-atlas auto-segmentation for liver and lung SBRT: comparison of two systems D. Dechambre 1 , P. Berkovic 2 , Z.L. Janvary 3 , N. Jansen 1 , A.P. Coucke 1 , V. Baart 1 , A. Gulyban 1 1 C.H.U. - Sart Tilman, Radiotherapy department, Liège, Belgium 2 U.Z. Leuven, Radiation Oncology, Leuven, Belgium 3 National Institute of Oncology, Radiotherapy department, Budapest, Hungary Purpose or Objective Atlas-based auto-segmentation has the potential to reduce the staff workload while improving delineation consistency. Our aim was to evaluate the volumetric accuracy of two commercial systems for lung and liver stereotactic treatment 1) while completing the atlas (learning curve), 2) using the full atlas (performance) and 3) determining dose volume histogram parameter (DVH) variations between the auto-generated and the reference contours. Material and Methods Forty random liver and lung cases were selected (CT, RT_DOSE, RT_STRUCT). Two systems were used, the single-atlas based Raystation ('RS’, v5.0.2, Raysearch) and the multi-atlas based RTx ('MIR’, v1.6.3, Mirada Medical). The 1-5 th case was used as base atlas. The learning phase was completed in an incremental way, where each new auto-contours was generated using an atlas consisting of all former cases, until the 20 th case. Performance was evaluated using the complete atlas on another 20 cases. Analysis included the Dice Similarity Coefficient (DSC), Jaccard index (JI), commonly contoured volumes (CCV), volumetric ratios (VR) and 95% of the Hausdorff distance (HD95%). Furthermore using the dose matrix, DVHs were generated for all volumes and the differences of relevant organs at risk specific parameters were compared. Results For volumetric comparison (DSC, JI, and CCV, VR, HD95%) liver and lung showed 12 vs. 1 out of 20 and 16 vs. 20 out of 35 parameters that improved from the learning to the performance stages for MIR vs. RS respectively (Table. 1).

parameter but the volume ratio. Similar results were obtained for the spinal canal except that RS was more accurate in estimating the VR (p=0.68). The two systems showed comparable results for the trachea. Esophagus and stomach showed overall poor agreement for both systems. MIR surpass RS for the Liver and both kidney for all the analyzed parameters. Root mean square values (RMS) of MIR vs. RS (Fig. 2) resulted in 0.23 vs. 0.31 Gy of D1.2cc for the spinal canal, for the trachea 0.99 vs. 1.1 Gy of D4cc, for great vessel 0.77 vs. 2.6 Gy of D10cc, for the right Kidney 6.17 vs. 9.78 Gy of D10cc and for the liver 1.43 vs. 1.87 Gy of D700cc. Only Heart D15cc and left Kidney D10cc RMS differences of 0.71 vs. 1.09 Gy and 0.49 vs. 1.15 Gy were statistically significant.

Conclusion Using multi-atlas (MIR) compared to single-atlas (RS) for liver/lung showed better in 16/22, similar in 3/11 and worse in 1/2 out of the 20/35 volumetric parameters. Lung atlas showed promising results that could open the door for a more intensive use in clinical routine while the discordances for liver cases could lead to large DVH parameters differences. Nevertheless, multi-atlas auto- segmentation resulted in a better agreement with references in both cases. EP-2120 Optimization of UTE MR sequence for MR-only image guided procedures in Hea and Neck S. Aouadi 1 , S. Paloor 1 , T. Torfeh 1 , A. Celik 2 , M. McGarry 1 , P. Petric 1 , P. Caparrotti 1 , H. Fayad 1 , R. Hammoud 1 , N. Al- Hammadi 1 1 National Center for Cancer Care & Research, Radiation Oncology, Doha, Qatar 2 GE Healthcare, Eastern Europe & EGM, Antalya, Turkey Purpose or Objective MR-only image guided procedures require replacing CT with MRI which has challenges for bones visualization and electron density values. Hence, Ultrashort Time Echo (UTE) sequence, designed to visualize tissues with short relaxation times, was investigated for bone visualization. The acquisition parameters of UTE sequence were optimized to maximize bones and soft-tissues contrast and were assessed for image guided radiotherapy (IGRT). Material and Methods A novel UTE sequence (3D-UTE-Cones), employing 3D cones trajectory, implemented on 1.5T Optima 450w MR scanner (GE Healthcare Milwaukee, WI, USA) was optimized for the GEM RT Open Head&Neck Suite using an anthropomorphic head phantom (CIRS model 603A). Repetition time (TR), Echo time (TE), receiver bandwidth (RBW), matrix size, field of view (FOV), slice thickness and the number of excitations (NEX) were adjusted to balance trade-offs between signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), spatial resolution and scan- time while limiting imaging artifacts. CNR between bones and soft-tissues was computed in image difference (DTE) between two different echo time TE1=0.028ms and

For the great vessel and left lung, MIR showed superior performance for all parameters but statistical significance was achieved for only half them. For the heart and right lung, MIR outperformed RS (p<0.05) for all