ESTRO 36 Abstract Book

S903 ESTRO 36 2017 _______________________________________________________________________________________________

the repeatability of semi-automatic contouring was also tested. Results A total of 73 TA were extracted on each contour. A strong disagreement was found between automatic SUVmax threshold contours and manual or semi-automatic contours in terms of both DICE and TA agreement (9/73 TA for HNC and 10/73 for pancreas pts with p- value>0.05,Figure 2). Instead, both the inter-observer as well as the agreement between manual and semi- automatic contour was relatively high, for both volume (median DICE=0.71,range=0.36-0.96) and TA extraction (72/73 with p-value>0.05 for both HNC and pancreas pts). A high intra-observer agreement and a high contour repeatability were found for manual contours (median DICE=0.75,range:0.13-0.92) and for the semi-automatic method for lesions with high uptake values (median DICE=0.95,range=0.42-1.00). No statistically significant difference was found among scanners (p-value=0.12).

geometrical distortion. The system imperfection is more problematic in MR-Linac with split magnet and less homogeneous magnetic field compared to diagnostic MR systems. SE DWI techniques can reduce the geometrical distortion with the penalty of longer imaging time. Split acquisition of fast spin-echo signals for diffusion imaging (SPLICE) is a DWI technique combined with modified spin echo approach in which is insensitive to the phase of the magnetization. A commercial DWI phantom designed by The Radiological Society of North America Quantitative Imaging Alliance (QIBA) with known Apparent Diffusion Coefficient (ADC) at ice temperature was used in order to determine the optimum ADC measurement sequence for future clinical development. Use of the phantom also allows spatially accurate assessment of geometric distortion compared to CT images acquired using GE Discovery CT 750 HD with Slice thickness of 1.25mm and Voxel size of 0.4883x0.4883x1.25 mm 3 DWI imaging was performed using SS EPI (TR/TE = 10000/115 ms) and SS SPLICE (TR/TE = 10000/99 ms) with voxel size = 1.72x1.72mm; slice thickness = 4mm; number of slices=25; and b values = 0, 500, 900, 2000 s/mm 2 . Results Qualitative assessment of the geometrical distortion shows significant improvement using SPLICE-DWI against EPI-DWI compared to CT images as shown in figure 1. Quantitative ADC measurement revealed a consistency between measured values using DWI-EPI sequence acquired on Diagnostic MRI system and MR-Linac system in room temperature. The measured values in room temperature are about 33% larger than ADC values measured in 0 o C which is in agreement with our previous experiments on diagnostic MRI systems. However, the measured ADC values using SPLICE have larger variations specifically in higher b-values.

Conclusion Almost the totality of the selected radiomic features were sufficiently robust against the delineation when using manual and semi-automatic methods, while threshold based methods resulted to be less robust. The satisfactory results with a semi-automatic PET contouring method suggests, for the two clinically situations considered in this work, possible promising applications for consistent and fast textural feature extraction in multi-centric studies. EP-1679 Preliminary functional imaging study on an integrated 1.5T MR-Linac machine M. Kadbi 1 , Y. Ding 2 , J. Wang 2 , C.D. Fuller 3 1 Philips, MR Therapy, Gainesville, USA 2 MD Anderson, Department of Radiation Physics, Houston, USA 3 MD Anderson, Department of Radiation Oncology, Houston, USA Purpose or Objective Diffusion-weighted imaging (DWI) is a promising technique in MR guided radiotherapy (MRgRT) to delineate the tumor, predict response to induction chemotherapy, response to radiation therapy, and has been demonstrated as a biomarker of recurrence. This is the first attempt to investigate the performance of DWI technique in an integrated MR-Linac which combines Philips 1.5T MRI with 7 MV photon beam Elekta Linear accelerator (Linac). Conventional EPI-based DWI was compared with Spin-Echo (SE)-based DWI and geometrical distortion of the sequences were benchmarked with CT images as reference for geometric fidelity. Material and Methods Clinical single-shot EPI-DWI sequence is a rapid imaging technique commonly used for functional imaging. However, EPI techniques are very sensitive to hardware and software imperfection (e.g. B0 inhomogeneity and eddy current) as well as susceptibility effect causing

Conclusion The SPLICE DWI showed improved spatial fidelity compared to EPI-DWI. This is particularly beneficial in MRgRT due to importance of geometrical fidelity. The SPLICE-DWI sequence needs further modifications and calibrations to achieve more accurate ADC measurement. EP-1680 Assessing tumour necrosis in lunvg cancer with dual energy CT quantitative imaging V. González-Pérez 1 , E. Arana 2 , J. Cruz 3 , M. Barrios 2 , F. Blázquez 1 , A. Bartrés 4 , L. Oliver 1 , V. Campo 1 , C. Bosó 1 , P. Sanamaría 5 , V. Crispín 1 1 Fundación Instituto Valenciano de Oncología, Servicio de Radiofísica y Protección Radiológica, Valencia, Spain 2 Fundación Instituto Valenciano de Oncología, Servicio de Radiología, Valencia, Spain 3 Fundación Instituto Valenciano de Oncología, Servicio de Anatomía Patológica, Valencia, Spain 4 Onkologikoa, Servicio de Radiofísica y Protección Radiológica, San Sebastián, Spain