ESTRO 37 Abstract book

S41

ESTRO 37

material I -values to propose a new set of elemental I - values and corresponding uncertainties, based on the experimental uncertainties of the published data and our uncertainty model. We evaluate the resulting uncertainties on the I -values and RSPs of 70 human reference tissues, taking co-variances between tissues and water into account. We then quantify resulting differences on particle beam ranges using Monte Carlo simulations. Results In comparison to ICRU suggested values, our analytical assignment scheme of elemental I -values describes the measured material I -values (quoted measurement uncertainties of 1% - 8%, depending on material) for liquids and solids with higher accuracy (RMS errors of 14.35% (ICRU 37) vs 0.93% (this work)). Using our elemental I -values, we calculate the I -value of water as (77.1 +/- 2.6) eV. This value is in good agreement with the ICRU 90 recommended value for liquid water of (78 +/- 2) eV. From the uncertainty model we calculate uncertainties on human reference tissue I -values of 2.21% - 3.34%, which translate into RSP uncertainties between 0.002% - 0.43%, with the highest uncertainty observed in adipose tissue (figure 1). We observe range differences of a 173 MeV proton beam in human tissues between 0.43 mm (adipose tissue) and 0.95 mm (cortical bone) comparing ICRU I -values versus our I -values, where we observe systematically shorter ranges using ICRU (figure 2). The resulting range uncertainty can be quantified between 0.2% and 0.5% depending on the tissue type, as compared to current estimates of 1.5%.

Conclusion We propose a set of elemental I -values, found via an analytical model, that are well suited for the use in human tissues in combination with the Bragg additivity rule. Our model establishes uncertainties on I -values which enables to quantify the resulting uncertainties on RSPs and particle range by taking co-variances into account. OC-0085 Improving CT calibration for proton range prediction by dual-energy CT based patient-cohort analysis P. Wohlfahrt 1,2 , C. Möhler 3,4,5 , W. Enghardt 1,2,6,7 , M. Krause 1,2,6,7,8 , E.G.C. Troost 1,2,6,7,8 , S. Greilich 3,4 , C. Richter 1,2,6,7 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 German Cancer Research Center DKFZ, Medical Physics in Radiation Oncology, Heidelberg, Germany 4 National Center for Radiation Research in Oncology NCRO, Heidelberg Institute for Radiation Oncology HIRO, Heidelberg, Germany 5 Heidelberg University, Department of Physics and Astronomy, Heidelberg, Germany 6 Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden, Dresden, Germany