ESTRO 38 Abstract book

S559 ESTRO 38

from DECT scans processed in syngo.via (Siemens Healthineers). Three radiation oncologists with different levels of experience in neuro-oncology delineated the postoperative tumor bed volume (TBV) and OARs (brainstem, parotid and lacrimal glands, eyes, lenses, optic nerves, and chiasm) on each dataset, at least two- weeks apart per patient. Relevant image information was blinded. The delineations on SECT datasets were repeated once to assess the intra-observer variability. Finally, the delineation was also performed on T1/T2 MR scans as clinical reference. The contour conformity was quantified by the Jaccard index (JI) and Hausdorff distance (HD) between the intersection and union of the respective contours (Fig. 1).

Figure 1: Upper left: Relation between predicted SPR w and measured SPR meas . Upper right: SPR w residual for measured tissue surrogates. Bottom: Cross-section through a phantom with seven tissue-equivalent inserts showing predicted SPR w . Conclusion In this study we investigated the ability of Philips IQon Spectral CT imaging technique to predict the SPR w for particle therapy more accurately using an image-based calculation of both ED and Z eff . Further comparative studies of SPR w maps derived from spectral CT to those determined by conventional CT imaging for RT planning and range measured in biological tissue samples will evaluate its potential in realistic tissue compositions. These results will be additionally presented at the conference. PO-1012 Dual-energy computed tomography for improved delineation in postoperative brain-tumor patients P. Wohlfahrt 1,2 , L. Agolli 3 , M. Krause 1,2,3,4,5 , K. Pilz 3 , C. Richter 1,2,3,4,6 , E.G.C. Troost 1,2,3,4,5,6 1 OncoRay - National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus - Technische Universität Dresden - Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany ; 2 Helmholtz-Zentrum Dresden-Rossendorf, Institute of Radiooncology - OncoRay, Dresden, Germany ; 3 Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus - Technische Universität Dresden, Dresden, Germany ; 4 German Cancer Consortium DKTK, partner site Dresden and German Cancer Research Center DKFZ Heidelberg, Dresden, Germany ; 5 National Center for Tumor Diseases NCT, partner site Dresden: German Cancer Research Center DKFZ Heidelberg - Faculty of Medicine and University Hospital Carl Gustav Carus at Technische Universität Dresden and Helmholtz Association / Helmholtz-Zentrum, ; 6 Both authors share last authorship, equal contribution, Dresden, Germany Purpose or Objective The clinical use of dual-energy CT (DECT) contributes to an improved accuracy in proton treatment planning compared to single-energy CT (SECT) as demonstrated in recent studies. A precise delineation of tumor volumes and organs at risk (OARs) is essential in particular for emerging high-conformal treatment techniques. Since DECT provides additional tissue information and allows for the generation of various tissue contrasts, we assessed its influence on the intra- and inter-observer delineation variability. Material and Methods Two cohorts of 10 postoperative brain-tumor patients each, receiving either a 120kVp SECT or 80/140kVp DECT scan with identical total dose, were evaluated. Pseudo- monoenergetic CT (MonoCT) datasets of 50, 60, 70 and 79keV, representing several tissue contrasts, were derived

Results The median inter-observer TBV conformity (Fig. 2A) was almost independent from the CT dataset (HD=6-9mm, JI=61-66%) and comparable to MR scans (HD=6-7mm, JI = 66-67%). The consistency of brainstem contours (Fig. 2B) was best at the lowest energy of MonoCT datasets (median HD=2.8mm, JI=81%). In contrast, the contour conformity of the parotid glands (Fig. 2C) gained slightly from an increased energy (median HD reduction of 0.6mm, JI increase of 1%) and also led to better results as MR scans. For these OARs, using the most suitable MonoCT instead of SECT resulted in smaller inter-observer variations. No relevant differences between SECT and MonoCT were determined for the other OARs, potentially due to their small volume. The intra-observer TBV variability obtained on SECT did not depend on clinical experience. However, the contouring of less experienced clinicians is more affected by different image contrasts introduced by MonoCT of different energies (Fig. 2D).