Abstract Book

S1147

ESTRO 37

biggest value was 2,19% for 40 keV. Table 2 presents results for SNR and noise. Conclusion In this study a significant difference between low contrast resolution, SNR and noise for different energies have been observed. For improve low contrast resolution, signal to noise ratio and noise dual energy CT can be used. The best results for every evaluated parameters were obtained for 60 keV. In future reconstructed monoenergetic scans for 60 keV will be analyzed for potentially clinical use for delineation tumor. EP-2089 Dual-energy CT enables dose calculation on scans with iodinated contrast agent C. Möhler 1,2,3 , P. Wohlfahrt 4,5 , N.H. Nicolay 2,6,7 , C. Richter 4,5,8,9 , S. Greilich 1, 1 German Cancer Research Center DKFZ, Medical Physics in Radiation Oncology, Heidelberg, Germany 2 National Center for Radiation Research in Oncology NCRO, Heidelberg Institute for Radiation Oncology HIRO, Heidelberg, Germany 3 Heidelberg University, Department of Physics and Astronomy, Heidelberg, Germany 4 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 5 Helmholtz-Zentrum Dresden - Rossendorf, Institute of Radiooncology - OncoRay, Heidelberg, Germany 6 Heidelberg University Hospital, Department of Radiation Oncology, Heidelberg, Germany 7 German Cancer Research Center DKFZ, Clinical Cooperation Unit for Radiation Oncology, Heidelberg, Germany 8 Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden, Dresden, Germany 9 German Cancer Consortium DKTK, partner site Dresden, Dresden, Germany Purpose or Objective Radiotherapy planning commonly requires an additional, ‘native’ CT scan for dose calculation if a contrast agent is used for tumor diagnostics and contouring. Iodinated contrast agents increase CT numbers (Hounsfield units) due to the large atomic number of iodine (Z=53), while electron density remains almost unchanged owing to its low concentration (Figure 1). With dual-energy CT (DECT), the impact of atomic number on CT image contrast can be removed, enabling the direct calculation of relative electron density (RED) for photon therapy and stopping-power ratio (SPR) for ion therapy, respectively. In this study, we are investigating the magnitude of the remaining impact of an iodinated contrast agent on DECT-derived RED/SPR and subsequent clinical treatment planning for both photon and ion therapy.

Material and Methods As a first step, the effect of the CT contrast medium Imeron® 300 (Bracco Imaging Deutschland GmbH, Germany) on RED/SPR determination was investigated in a dilution series over a range of iodine concentrations between 0.3 and 300 mg/ml. CT images were acquired on a Somatom Definition Flash dual-source CT scanner (Siemens Healthineers, Forchheim, Germany) in single- energy (SECT, 120 kVp) and dual-energy (DECT, 80/140Sn kVp) scan mode. RED and SPR images were obtained (a) from SECT datasets by applying the respective calibrated Hounsfield look-up table and (b) from DECT datasets using the software application syngo.CT Rho/Z (Siemens) and an SPR calculation scheme previously validated by the authors in phantoms, biological material and patients. Results Calculating RED/SPR from a DECT dataset with typical contrast enhancement (max. 160 HU at 120 kVp corresponding to 6 mg iodine per milliliter) could limit the impact on both RED and SPR to 1% compared to 5-10% when using a contrast-enhanced SECT image (Figure 2). Consequently, dose calculation could be performed directly on DECT-derived RED/SPR images.