ESTRO 36 Abstract Book

S916 ESTRO 36 2017 _______________________________________________________________________________________________

features in the two CT modalities were compared based on the intraclass correlation coefficient (ICC1). The prognostic value of both modalities was assessed using univariable Cox regression. Additionally, in a previous analysis we built a local tumor control prognostic model based on contrast enhanced CT images of 93 patients. It comprised three radiomic features (HHH large zone high grey-level emphasis, LLL sum entropy and LLH difference variance). The performance of this model was comparatively tested in the two new CT datasets. Results 118 out of 693 radiomic features showed a good agreement between native CT and contrast enhanced CT imaging (ICC>0.8). None of the intensity features was stable and only 7 of the texture features in the non transformed map. In the univariable Cox regression 96 and 107 radiomic features in the native and contrast-enhanced CT images had a significant influence on the local control, respectively. 62 parameters were prognostic in both modalities, but only half of them showed a good agreement between native and contrast enhanced CT images (ICC>0.8). Two out of the three features of our previously developed model were stable in respect to the administration of contrast agent in the CT images (ICC>0.8). However, our model was only predictive for the parameter set derived from the contrast enhanced CT images (CI=0.69, p=0.013). Based on this, patients could be divided into 2 risk groups (p=0.02, Figure 1).

A major source of error in quantitative PET/CT of lung cancer tumors is respiratory motion. Regarding variability of PET textures features (TF), the impact of respiratory motion has not been properly studied with experimental phantoms. The primary aim of this work was to evaluate the current use of PET texture analysis for heterogeneity characterization in lesions affected by respiratory motion. Material and Methods Twenty-eight heterogeneous lesions were simulated by a mixture of alginate and 18F- fluoro-2-deoxy-D-glucose (FDG). Sixteen respiratory patterns were applied. Firstly, the TF response for different heterogeneous phantoms and its robustness with respect to the segmentation method were calculated. Secondly, the variability for TF derived from PET image with (gated, G-) and without (ungated, U- ) motion compensation was analyzed. Finally, TF complementarity was assessed. Results In the comparison of TF derived from the ideal contour (VOI_Ideal) with respect to TF derived from 40%-threshold (VOI_40%) and adaptive-threshold (VOI_COA) PET contours, 7/8 TF showed strong linear correlation LC (p<0.001, r>0.75), despite a significant volume underestimation. Independence on lesion movement (LC in 100% of the combined pairs of movements, p<0.05) was obtained for 1/8 TF with U-image (width of the volume- activity histogram, WH) and 4/8 TF with G-image (WH and energy ENG, local-homogeneity LH and entropy ENT, derived from the co-ocurrence matrix). Apart from WH (r>0.9, p<0.001), no one of these TF has shown LC with Cmax. Complementarity was observed for the TF pairs: ENG-LH, CONT-ENT and LH-ENT. Conclusion In conclusion, effect of respiratory motion should be taken into account when heterogeneity of lung cancer is quantified on PET/CT images. Despite inaccurate volume delineation, TF derived from 40% and COA contours could be reliable for their prognostic use. The TF that exhibited simultaneously added value and independence of lesion movement were ENG and ENT computed from G-image. Their use is therefore recommended for heterogeneity quantification of lesions affected by respiratory motion. EP-1699 The simulation study of position and biology of target with PET in high energy X-Ray irradiation Q. Zhang 1 1 Topgrade Medical - Yiren Hospiatl, Radio- therapy center, BEIJING, China Purpose or Objective To study the possibility of in situ verification of dose distributions and position in radiation therapy with PET imaging based on the activity distribution of 11 C and 15 O produced via photonuclear reactions in patient irradiated by 45MV X ray from the LA45 accelerator. Material and Methods The method is based on the photonuclear reactions in the most elemental composition 12 C, 16 O in body tissues irradiated with high-energy photons with energies up to 45 MeV, resulting primarily in 11 C and 15 O, which are positron emitting nuclei. The induced positron activity distributions were obtained with a PET scanner in the same room of a LA45 accelerator (Top Grade Medical, Beijing, China). The activity distributions of 11 C and 15 O were used to verify the dose distributions and position in tarfet as delivered by the LA45 accelerator. The experiments were performed with a brain phantom. Radiation beams were delivered to the phantom according to realistic radiation therapy treatment plans. The phantom was immediately transfer to PET anfd was scanned on the PET after irradiation. The scans were performed at 20 minutes and 2-5 minutes after irradiation for 11 C and 15 O, respectively. The interval between the two scans was 20 minutes. The activity distributions of 11 C and 15 O within the irradiated volume can be separated from each other because the half-life of 11 C and 15 O is 20

Figure 1: The patients could be divided into two risk groups (p=0.02) based on the radiomics of contrast enhanced CT images and the model derived from our previous training cohort (contrast enhanced CTs). The splitting was not significant for radiomics of native CT images. Conclusion Radiomic models can be built on a mixed set of native and contrast-enhanced CT images but with a reduced number of suitable radiomic features. A model built exclusively with contrast enhanced CT images cannot be validated in a set of native CT Images. EP-1698 Impact of motion and segmentation on PET texture features: evaluation with heterogeneous phantoms. M. Carles 1,2 , I. Torres-Espallardo 2 , D. Baltas 1 , U. Nestle 1 , L. Martí-Bonmatí 2 1 University Medical Center, Department of Radiation Oncology, Freiburg, Germany 2 Hospital Universitario y Politécnico La Fe, Medical Imaging, Valencia, Spain

Purpose or Objective