ESTRO 36 Abstract Book

S81 ESTRO 36 _______________________________________________________________________________________________

presence of metal artefacts. To eliminate left-right bias, each combination was shown twice.

Results VMDE images reconstructed at energies in the range 60 to 70 keV showed improved CNR for all soft tissue regions when compared to the standard CBCT. On average, the reconstruction energy corresponding to the maximum CNR improvement is 65.5 ± 2.4 keV. The increase in maximum CNR varied from 29% to 78%. The clinical observer comparison gave a series of rankings for each image series for each patient (see table 1). Using signed rank Wilcoxon comparison test, the observers found the VMDE images at 65 keV preferable to the standard CBCT image. The p-value was found to be < 0.01, where p < 0.05 is considered significant. An estimate of inter- observer variability test was done with Fleiss’ kappa and found to be moderate with a κ-value of 0.47, where values above 0.4 is considered acceptable and 1 is perfect agreement. Occasionally, an observer ranked the 75 keV reconstruction as the most preferable image while the overall preferred image was the 65 keV reconstruction. Except in the case of patient one where the standard CBCT image was ranked the highest of all.

Conclusion We have shown that an a priori scatter correction algorithm for CB projections improves CBCT image quality on both photon- and proton therapy gantries, potentially opening for CBCT-based image/dose-guided proton therapy. OC-0159 Dual energy CBCT increases soft tissue CNR ratio and image quality compared to standard CBCT in IGRT M. Skaarup 1 , D. Kovacs 1 , M.C. Aznar 1 , J.P. Bangsgaard 1 , J.S. Rydhög 1 , L.A. Rechner 1 1 The Finsen Center - Rigshospitalet, Clinic of Oncology, Copenhagen, Denmark Purpose or Objective We investigate a method for enhancing soft tissue contrast to noise ratio (CNR) and clinical image quality of cone- beam computed tomography (CBCT) by using a dual energy CBCT protocol. Material and Methods Nine patients were scanned using a standard CBCT protocol of either 100 or 125 kVp and a DE-CBCT protocol of two separate scans of 80 and 140 kVp respectively. Other scan parameters were identical and total radiation dose was kept at a similar level for both protocols. Virtual monochromatic dual energy (VMDE) images were reconstructed using a linear mix of the 80 and 140 kVp scan. The weight, with which the two images were combined, was calculated based on known attenuation coefficients of two basis materials at a specific monochromatic energy. A linear combination of these can be used to express the attenuation coefficients of the 80 and 140 kVp scan at that same monochromatic energy. To find the optimal virtual reconstruction energy for soft tissue imaging, multiple reconstructions were done for energies in the range 40- 180 keV. CNR measurements were performed on both standard CBCT and VMDE images for a number of different tissue combinations, e.g. contrast between tumour-fat, tumour- surrounding tissue, muscle-fat, rectum-surrounding tissue, parotid-fat, seminal vesicle-surrounding tissue and lung-heart (see figure 1 for an example). In addition, 5 experienced observers conducted a blinded ranking between VMDE images (reconstructed at 55, 65, 75 and 100 keV) and the standard CBCT images, i.e. five image series per patient. For each combination of image series the observers were asked to compare the images side-by- side, focusing on soft tissue image quality as well as

Conclusion VMDE images can increase soft tissue contrast and improve clinical image quality for image-guided radiotherapy compared to the standard CBCT protocol. OC-0160 Radiomics Features Harmonization for CT and CBCT in Rectal Cancer R. Luo 1 , J. Wang 1 , H. Zhong 1 , J. Gan 1 , P. Hu 1 , L. Shen 1 , W. Hu 1 , Z. Zhang 1 1 Fudan University Cancer Hospital, Radiation Oncology, Shanghai, China Purpose or Objective Inter-scanner variability can lead to degradation of radiomics model quality. Therefore, feature harmonization is necessary for consistent findings in radiomics studies, especially for multi-institution studies. The purpose of this study is to establish harmonization