ESTRO 37 Abstract book

ESTRO 37

S534

PO-0969 Inter-center variability of CT-to-SPR conversion in particle therapy: Survey-based evaluation V.T. Taasti 1 , C. Bäumer 2 , C.V. Dahlgren 3 , A.J. Deisher 4 , M. Ellerbrock 5 , J. Free 6 , J. Góra 7 , A. Kozera 8 , A.J. Lomax 9 , L. De Marzi 10 , S. Molinelli 11 , B.K. Teo 12 , P. Wohlfahrt 13 , J.B.B. Petersen 1 , L.P. Muren 1 , D.C. Hansen 1 , C. Richter 13 1 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark 2 Westdeutsches Protonentherapiezentrum Essen gGmbH, Department of Medical Physics, Essen, Germany 3 University of Uppsala- Akademiska Sjukhuset, Section of Hospital Physics, Uppsala, Sweden 4 Mayo Clinic, Department of Radiation Oncology, Rochester, USA 5 Heidelberg Ion Beam Therapy Center HIT, Department of Medical Physics, Heidelberg, Germany 6 University Medical Center Groningen- University of Groningen, Department of Radiation Oncology, Groningen, The Netherlands 7 EBG MedAustron GmbH, Department of Medical Physics, Wiener Neustadt, Austria 8 Cyclotron Centre Bronowice Institute of Nuclear Physics Polish Academy of Sciences CCB IFJ PAN, Oncology, Kraków, Poland 9 Paul Scherrer Institut, Center for Proton Therapy, Villigen, Switzerland 10 Institut Curie- Centre de protonthérapie d’Orsay, Department of Radiation Oncology, Orsay, France 11 CNAO, Department of Medical Physics, Pavia, Italy 12 University of Pennsylvania, Department of Radiation Oncology, Philadelphia, USA 13 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 Purpose or Objective To assess the inter-center variability of the conversion between CT number and particle stopping power ratio (SPR), a survey-based evaluation was carried out in the framework of the European Particle Therapy Network (EPTN). The conversion is applied to treatment planning CTs to finally derive the proton range in patients. Currently, CT scan protocols for treatment planning are not standardized in image acquisition and reconstruction parameters. Hence, the CT-to-SPR conversion (Hounsfield look-up table, HLUT), depending on the former parameters, has to be defined by each center individually. Aiming to access the current status of inter- center differences, this investigation is a first step towards better standardization of CT-based SPR derivation. Material and Methods A questionnaire was sent out to particle therapy centers involved in the EPTN and two centers in the United States. The questionnaire asked for details on CT scanners, acquisition and reconstruction parameters, the calibration and definition of the HLUT, as well as body- region specific HLUT selection. It was also assessed whether the influence of beam hardening (BH) on the HLUT was investigated and if an experimental validation of the HLUT was performed. Furthermore, different future techniques were rated regarding their potential to improve range prediction accuracy.

Results Twelve centers completed the survey (10 in Europe, 2 in the US). Scan parameters, in particular reconstruction kernel and beam hardening correction, as well as the HLUT generation varied widely between centers. Eight of the twelve centers applied a stoichiometric calibration method, while three defined the HLUT entirely based on tissue substitutes, and one center used a combination of both. All facilities performed a piecewise linear fit to convert CT numbers into SPRs, but the number of line segments used varied from 2 to 11 (Table 1). Nine centers had investigated the influence of BH, and seven of them had evaluated the size dependence of their conversion. All except one center had validated their HLUT experimentally, but the validation schemes varied widely. A few things were though found to be common for most centers: CT scans were most commonly acquired at 120 kVp, all centers individually customized their CT- to-SPR conversion, and dual energy CT was seen as a promising technique to improve SPR prediction (Figure 1).