ESTRO 37 Abstract book

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ESTRO 37

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.

constraint below 12min and a flip angle FA=12° were chosen. N small spheres ( r=4mm ), selected in bones and soft-tissues regions, were used for CNR estimation. Synthetic CT (sCT) was generated by an automatic segmentation of DTE into bones, soft-tissues, and air and then setting bulk Hounsfield units to the voxels. It was compared with the reference CT. CBCT and kv images (lateral, anteroposterior) of the head phantom were acquired using the Linear Accelerator for the assessment of MR-only IGRT. The results of 3D/3D and 2D/2D auto- matching, using the TPS Eclipse TM v11.0 (Varian Medical System), were compared between sCT and CT or between derived DRRs. Results The optimal SNR for good head region coverage was obtained for an isotropic resolution 1.3x1.3x1.3mm 3 , a matrix size of 186x186x100, NEX=2, and RBW=62.5 KHz with respect of chemical shift artefacts below 1mm. Figure 1.d shows variation of CNR according to TR and TE2 values. The scan-time increased with TR values. TE2=4.4ms, TR=9ms gave the best CNR/scan-time compromise with CNR=4.66 and scan-time=7:25min and was used for studying the clinical feasibility. sCT had a mean absolute error MAE=113HU and a Dice coefficient in bones DI bone =0.8. Figure 1.a, 1.b, 1.c show the results of the fusion of registered sCT or DRR sCT with the linac’s images. The largest deviations of sCT/CBCT registration parameters were <1.1mm and <0.7° for translation and rotation parameters respectively. For couch shifts in vertical, longitudinal and lateral directions, the largest deviations were <0.8mm after DRR sCT /kV images automatching.

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 Head&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 TE2=3.5, 4.4, 8.8, 9.5ms. To acquire the data, TR value as 8.6,9, 13.4, 14.1ms when possible, a scan-time

Conclusion 3D-UTE-Cones were optimized and tested for clinical implementation of MRI-only IGRT. In future work, the feasibility will be demonstrated on real patients with brain or H&N tumor. EP-2121 Serial Parotid Gland Radiomic-based Model Predicts Post-Radiation Xerostomia in Oropharyngeal Cancer H. Elhalawani 1 , A.S.R. Mohamed 1 , A. Kanwar 1 , A. Dursteler 1 , C.D. Rock 1 , S.E. Eraj 1 , M. Meheissen 1 , S. Volpe 1 , P. Yang 1 , R. Granberry 1 , R. Ger 1 , X. Fave 1 , L. Zhang 1 , J. Yang 1 , G.E. Marai 2 , D. Vock 3 , G. Canahuate 4 , D. Mackin 1 , L. Court 1 , G.B. Gunn 1 , A. Rao 1 , C.D. Fuller 1 1 The University of Texas- MD ANderson Cancer Center, Radiation Oncology, Houston, USA 2 University of Illinois at Chicago- Chicago- Illinois- USA, Computer Science, Chicago, USA