ESTRO 36 Abstract Book

S912 ESTRO 36 _______________________________________________________________________________________________

EP-1673 Electron-density assessment using dual-energy CT: accuracy and robustness C. Möhler 1,2 , P. Wohlfahrt 3,4 , C. Richter 3,4,5,6 , S. Greilich 1,2 1 German Cancer Research Center DKFZ, Division of 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 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 4 Helmholtz-Zentrum Dresden-Rossendorf, Institute of Radiooncology, Dresden, Germany 5 Department of Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden, Dresden, Germany 6 German Cancer Consortium DKTK, Dresden, Germany Purpose or Objective Current treatment planning for essentially every external radiation therapy (photons, electrons, protons, heavier ions) is not able to account for patient-specific tissue variability or non-tissue materials (e.g. implants, contrast agent) which can lead to considerable differences in dose distributions (figure 1). This is due to the conversion of CT numbers to electron density or stopping power using a heuristic Hounsfield look-up table. In contrast, dual- energy CT (DECT) allows for a patient-specific determination of electron density – the only (most important) parameter influencing photon (ion) dose distributions. Among the many algorithms proposed for this purpose, a trend towards increased complexity is observed, which is not necessarily accompanied by increased accuracy and might at the same time militate against clinical implementation. Here, we therefore investigated the performance of a seemingly simple linear-superposition method (Saito, 2012, Hünemohr et al., 2014).

White, 1986). A clear separation from CT-related sources of uncertainty (e.g. noise, beam hardening) is hereby crucial for a conclusive assessment of accuracy. Secondly, we tested the proposed calibration method on published DECT measurements of typical tissue-surrogate phantoms and evaluated its uncertainty. Results The methodological uncertainty of electron-density assessment for the alpha-blending method was found to be below 0.15% for arbitrary mixtures of human tissue. In the case of small abundance of high-Z elements, electron- density results are positively biased, e.g. 0.5% for thyroid containing 0.1% iodine (Z=53) by mass, which is due to the K edge of the photoelectric effect. The calibration parameters obtained from various published data sets, showed very little variation in spite of diverse experimental setups and CT protocols used. The calibration uncertainty was found to be negligible for soft tissue while it was dominated by beam hardening effects for bony tissue. Conclusion The alpha-blending approach for electron-density determination shows universal applicability to any mixture of human tissue with a very small methodological uncertainty (< 0.15%); and a robust and bias-free calibration method, which is straightforward to implement. We conclude that further refinement of algorithms for DECT-based electron-density assessment is not advisable. EP-1674 Experimental investigation of CT imaging approaches to deal with metal artefacts in proton therapy S. Belloni 1,2 , M. Peroni 1 , S. Safai 1 , G. Fattori 1 , R. Perrin 1 , M. Walser 1 , T. Niemann 3 , R.A. Kubik-Huch 3 , A.J. Lomax 1 , D.C. Weber 1,4,5 , A. Bolsi 1 1 Paul Scherrer Institut, Center for Proton Therapy, Villigen PSI, Switzerland 2 University of Bologna, Department of Physics and Astronomy, Bologna, Italy 3 Cantonal Hospital Baden, Department of Radiology, Baden, Switzerland 4 Inselspital, Radiation Oncology, Bern, Switzerland 5 University Hospital Zurich, Radiation Oncology, Zurich, Switzerland Purpose or Objective Metal implants are challenging for proton therapy, mainly because of beam hardening artefacts severely compromising image quality of the planning CT. In fact, they result in non-negligible uncertainties in Stopping Power (SP) evaluation and significantly affect VOI delineation accuracy. The aim of this study was to compare different approaches to minimize the artefacts: a manual approach based on delineation of the visible artefacts, which was developed and is used clinically at the Center for Proton Therapy (PSI), and the new tools recently introduced in CT, such as SIEMENS Iterative Metal Artefact Reduction (iMAR) and Sinogram Affirmed Iterative Reconstruction (SAFIRE). Moreover, an experimental verification of direct SP calculation from Dual Energy (DE) images with iMAR has also been considered. Material and Methods A clinical treatment of a cervical chordoma patient was reproduced on a head and neck anthropomorphic phantom, which presents metal implants (titanium screws and cage) in the area where the PTV was defined. An IMPT plan with two anterior oblique and two posterior oblique fields (dose per fraction 2 GyRBE) was optimized and calculated on 7 different CTs which corresponded to the different imaging approaches: no correction of artefacts, manual correction, iMAR (each of these reconstructed using Filtered Back Projection (FBP) and SAFIRE) and DE

Material and Methods Key feature of the studied approach is a parameterization of the electron density, given by 'alpha blending” of the two DECT images. The blending parameter can be obtained by empirical calibration using a set of bone tissue surrogates and a linear relationship between relative photon absorption cross sections of the higher and lower voltage spectrum. First, this linear relation was analyzed to quantify the purely methodological uncertainty (i.e. with ideal CT numbers as input), based on calculated spectral-weighted cross sections from the NIST XCOM database for tabulated reference tissues (Woodard and