ESTRO 2020 Abstract Book

S390 ESTRO 2020

OC-0698 First-in-man validation of CT-based stopping- power prediction using prompt-gamma range verification J. Berthold 1,2 , C. Khamfongkhruea 1 , A. Jost 1,3 , J. Petzoldt 4,5 , J. Thiele 6 , T. Hölscher 6 , P. Wohlfahrt 1,2,7 , C. Hofmann 8 , G. Pausch 1,2 , G. Janssens 4 , S. Julien 4 , C. Richter 1,2,6,9 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 Beuth Hochschule für Technik, Berlin, Germany ; 4 Ion Beam Applications SA, Louvain-la-Neuve, Belgium ; 5 Now with Thermo Fisher Scientific Bremen GmbH, Bremen, Germany ; 6 Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden, Dresden, Germany ; 7 Now with Massachusetts General Hospital and Harvard Medical School, Department of Radiation Oncology, Boston, USA ; 8 Siemens Healthineers, Forchheim, Germany ; 9 German Cancer Consortium DKTK- partner site Dresden, Germany and German Cancer Research Center DKFZ, Heidelberg, Germany Purpose or Objective Currently, the uncertainty in CT-based range prediction is substantially impairing the accuracy of particle therapy. Direct determination of stopping-power ratio (SPR) from dual-energy CT (DECT) has been proposed (DirectSPR) and initial validation studies in phantoms and biological tissues have proven a superior accuracy. However, a validation of range prediction in patients has not been achieved by any means. Here, we present the first verification of CT-based proton range prediction in patients, using prompt-gamma imaging (PGI). Material and Methods A PGI slit camera system of improved positioning accuracy, using a floor-based docking station, was developed. Its accuracy and positioning reproducibility were determined with x-ray and PGI measurements. The PGI system was clinically applied to monitor absolute proton ranges for a 1.5 GyE field during hypo-fractionated treatment of 3 prostate-cancer patients using pencil beam scanning (PBS) (Fig. 1). Per patient 3 fractions were monitored. For all monitored fractions, in-room control-CT (cCT) scans were acquired in treatment position enabling PGI-based spot- by-spot range analysis for the actual patient anatomy: The PGI measurements were compared to simulations of the expected PGI signal based on the respective cCT. Three different SPR prediction models were applied in the simulation: A standard CT-number-to-SPR conversion (Std- HLUT), a HLUT optimized with DECT-derived SPR information (Adapt-HLUT), and the directly voxel-wise calculated SPR based on the input from DECT (DirectSPR). To verify range prediction in patients, the histogram of PGI-derived range shifts from all PBS spots was analyzed concerning its Gaussian mean – acting as surrogate for the accuracy of the respective range prediction method. It is independent from random uncertainty contributions (e.g. positioning, statistical uncertainty in shift determination).