ESTRO 2023 - Abstract Book

S75

Saturday 13 May

ESTRO 2023

The time allocation for the PSQA task was less than 2 minutes for (i), up to 20 and 10 minutes for (ii) and (iii), respectively. The times between the MR and CT simulation for (iv) were patient dependent and limited to 30 minutes. The dCT was the only method including the artificial implants in the electron density map. Figure 2 reports the observed differences of the GTV mean dose between the calculation performed on the reference sCT and the PSQA. Deviations within 2% were observed only for (iii) and (iv) across the whole cohort. In absence of large air pockets and lung tissue within the beam path, (i) and (ii) also provided valuable results. Equivalence testing (p=0.05) of multiple DVH parameters confirmed that the independent NN and dCT can provide PSQA applicable to the 1% level for the sub-groups (a) and (b), 1.5% for (c) and 2% for (d).

Conclusion Four independent strategies for performing PSQA of sCT in the context of MR-only radiotherapy were investigated. The use of an independent NN generating as sCT for dose verification was demonstrated to be applicable for PSQA if testing DVH parameters to the 2% equivalence level. The PSQA task was performed within 10 minutes for each patient, which meets the requirements for the integration within the planning workflow. OC-0115 Consensus guide on CT-based prediction of stopping-power ratio using a Hounsfield look-up table V. Taasti 1 , N. Peters 2 , A. Bolsi 3 , C. Vallhagen Dahlgren 4 , M. Ellerbrock 5 , C. Gomà 6 , J. Góra 7 , P. Cambraia Lopes 8 , I. Rinaldi 9 , K. Salvo 10 , I. Sojat Tarp 11 , A. Vai 12 , T. Bortfeld 13 , A. Lomax 14 , C. Richter 2 , P. Wohlfahrt 13 1 Department of Radiation Oncology (MAASTRO), GROW – School for Oncology, Maastricht University Medical Centre+, Maastricht, The Netherlands; 2 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; 3 Paul Scherrer Institute, Center for Proton Therapy, Villigen, Switzerland; 4 The Skandion Clinic, Department of Medical Physics, Uppsala, Sweden; 5 Heidelberg Ion-Beam Therapy Center (HIT), Department of Radiation Oncology, Heidelberg University Hospital, Heidelberg, Germany; 6 Department of Radiation Oncology, Hospital Clínic de Barcelona, Barcelona, Spain; 7 EBG MedAustron GmbH, Division of Medical Physics, Wiener Neustadt, Austria; 8 Holland Proton Therapy Center, Department of Medical Physics, Delft, The Netherlands; 9 Department of Radiation Oncology (Maastro), GROW School for Oncology, Maastricht University Medical Centre+, Maastricht, The Netherlands; 10 Algemeen Ziekenhuis Sint-Maarten, Department of Radiotherapy, Duffel, Belgium; 11 Danish Centre for Particle Therapy, Aarhus University Hospital, Aarhus, Denmark; 12 Radiotherapy Department, Center for National Oncological Hadrontherapy (CNAO), Pavia, Italy; 13 Massachusetts General Hospital and Harvard Medical School, Department of Radiation Oncology, Boston, MA, USA; 14 Center for Proton Therapy, Paul Scherrer Institute, Villigen, Switzerland Purpose or Objective Studies within the European Particle Therapy Network (EPTN) have shown a large variation in the estimation of proton stopping-power ratio (SPR) from computed tomography (CT) scans across European proton centres. To standardise the SPR prediction process, we present a step-by-step guide on the Hounsfield look-up table (HLUT) specification process. This consensus guide was created within the ESTRO Physics Workshop 2021 on CT in radiotherapy in a joint effort with the EPTN The HLUT specification procedure is divided into six steps (Figure 1): 1) phantom setup, 2) CT scanning, 3) CT number extraction, 4) SPR determination, 5) HLUT specification, 6) HLUT evaluation. For each step, considerations and recommendations are given based on literature and additional experimental evaluations. Appropriate phantom inserts are tissue-equivalent for both X-ray and proton interactions and are scanned in head- and body-d phantoms to mimic different beam hardening conditions. Soft tissue inserts can be scanned together, while bone inserts are scanned individually to avoid imaging artefacts. CT numbers are extracted in material-specific regions-of-interest covering the inner 70% of each phantom insert in-plane and several axial CT slices in scan direction. For an appropriate HLUT specification, the SPR of phantom inserts is experimentally determined in proton range measurements at an energy >200 MeV, and the SPR of tabulated human tissues is computed stoichiometrically at 100 MeV. By including both phantom inserts and tabulated human tissues in the HLUT specification, the influence of the respective dataset-specific uncertainties are mitigated and thus the HLUT accuracy is increased. Piecewise linear regressions are performed between CT numbers and SPRs for four individual tissue segments (lung, adipose, soft tissue and bone) and then connected with straight lines. A thorough but simple validation is finally performed. Work Package 5 (WP5). Materials and Methods