Abstract Book

ESTRO 37

S533

0.7%(-12.9% to 20.2%) and 16.3% (-33.8% to 66.4%), respectively. The amplitude deviation was in average within 2 mm (figure 1). However, underestimations up to 6.3mm were observed. No correlations were found between acquisition parameters and volume or amplitude deviations.

= 0.1% and Z eff

= 3.4% compared to

common insert, with ρ e

= 0.8% and Z eff

= 6.0% for Saito et al.

ρ e

Conclusion Iterative reconstruction CBCT combined with dual energy acquisitions allows for a significant dose reduction (at least a factor 10) by decreasing the required count statistics. The ρ e and Z eff values required for segmentation and dose calculation can be obtained without a loss of accuracy. Further investigations will focus on the use of these images for treatment planning and direct CBCT based dosimetry applications. 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.

Conclusion Our data suggest that the expiration phase is the most accurate phase to define the tumour volume and should therefore be preferred for GTV delineation when using a mid-position, gating or tracking strategy. The large variation found among the institutions indicated that further improvements in 4DCT imaging are possible and local 4DCT QA could be improved. PO-0968 Dual energy and iterative CBCT recons- truction allows reducing patient dose M. Gilles 1 , D. Benoit 2 , J. Bert 2 , A. Iborra 2 , U. Schick 3 , O. Pradier 3 , H. Fayad 1 , N. Boussion 4 , D. Visvikis 2 1 UBO - INSERM UMR 1101, LaTIM, Brest, France 2 INSERM UMR 1101, LaTIM, Brest, France 3 CHRU Morvan, Radiotherapy, Brest, France 4 CHRU Morvan - INSERM UMR 1101, LaTIM, Brest, France Purpose or Objective CBCT are increasingly used during radiotherapy treatment for patient positioning and adaptive treatment. These images induce a dose increase to the patients. The idea of this work is to reduce the delivered dose with no loss of information using dual energy CBCT acquisitions in combination with an iterative reconstruction algorithm. Material and Methods We first explored iterative reconstructed CBCT images (maximum likelihood gradient ascent optimization and patchwork-based reconstruction from Van Slambrouck and Nuyts, 2 iterations, 360 projections) to determine how much we can decrease count statistics while keeping distinguishable phantom inserts. We then used the dual energy method described by Saito et al. (2017) on these images in order to verify if the materials’ electron density (ρ e ) and effective atomic number (Z eff ) can be accurately retrieved from such low dose images. We used 80kVp and 140kVp (38kV and 65kV) for the CBCT acquisitions, in combination with materials mimicking CatPhan phantom inserts and body tissues. Results The number of counts considered varied from 40 to 2000 per pixel. Although 1000 counts resulted in correct insert distinction by plot profiles, we chose to use the 2000 counts images (10 to 20 times less than an actual state of the art clinical CBCT) to reduce overall image noise. We obtained similar errors for ρ e and Z eff than Saito et al.: 0.5% ± 0.4 vs. 0.4% ± 0.4 for ρ e and 2.0% ± 1.2 vs. 2.2% ± 1.6 for Z eff considering all inserts (calibration and body tissue like). The details of the results are summarized in the table. We obtained smaller errors for our adipose