ESTRO 36 Abstract Book

S916 ESTRO 36 _______________________________________________________________________________________________

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 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.

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 5 Fundación Instituto Valenciano de Oncología, Servicio de Radioterapia, Valencia, Spain Purpose or Objective To assess if dual energy computed tomography (DECT) quantitative imaging can distinguish necrotic tumours in lung cancer. Material and Methods From July 2013 to June 2016, 83 patients who underwent a DECT study were reviewed for their lung tumour necrosis status (33 positive; 50 negative). Lesion size varied considerably: the mean lesion volume was 15 cm 3 (range 0.05-138 cm 3 ). Malignant lesions were predominantly adenocarcinoma (77.1%), squamous cell carcinoma (13.2%) and metastases (7.2%). DECT examination was performed on a Discovery CT 750 HD scanner (GE Healthcare, WI, USA). Patients were injected with 1.35 ml/kg of body weight of non-ionic iodinated contrast material at 4 ml/s (Iopamidol, 300 mg/ml; Bracco, Italy). A Gemstone Spectral Imaging (GSI) DECT exam of the entire chest was performed at arterial phase. Lesion volume was semi-automatically segmented using Dexus lung nodule function (ADW4.6; GE Healthcare, USA) by two radiologists. Images for quantitative iodine content ρ I (mg/cm 3 ) and effective atomic number (Z eff ) were reconstructed. Maximum, mean and standard deviation values were recorded for both parameters and for conventional HU image. Lesion volume and diameter were also registered. Inter- and intra-observer intraclass correlation coefficient (ICC) was studied. Bilateral statistical analysis was performed using the Mann-Whitney U test. Due to multiple comparisons, Bonferroni adjustment was made and significance was set at p < 0.007. Receiver operating characteristic (ROC) curves were generated and diagnostic capability was determined by calculating the area under the ROC curve (AUC). The licensed statistical software package SPSS 20 (IBM, Somers, NY, USA) was used. Results Reproducibility of intraobserver lung lesion the ICC was 0.95 (CI 95% 0.80–0.98) and interobserver ICC was 0.92 (CI 95% 0.70–0.98). The bivariate analysis for distinguishing necrotic from non- necrotic lesions revealed statistically significant differences. Larger lesions presented more necrosis than smaller ones, as previously known in the literature. Values for p, AUC and its 95% confidence level interval are shown in Table 1.

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

Box-whisker and ROC plots are displayed in Fig. 1 for mean Z eff variable, which presented highest AUC (0.890). Mean