Abstract Book

S42

ESTRO 37

Results The level of quenching at the Bragg peak varies from 80%, 36%, 22%, 15%, and 13% for the corresponding proton beam energies 85.6, 100.9, 124, 144.9, and 161.6 MeV (figure 1). The calculated light using Birks scintillation model fits well compared to the measured light to within 5% for all proton beam energies, except for the lowest beam energy (85.6 MeV), which resulted in over 10% difference around the Bragg peak. Conclusion A quenching correction factor can be extracted from Birks quenching parameter and applied to the measured light to determine absorbed dose for proton beams. Further improvements and modifications to the existing ionization quenching models are warranted to improve the agreements especially at the lower energies.

optimization. We include recent literature on measured 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%.

Figure 1 . (Top) Central axis depth-dose profiles for the Monte Carlo simulated dose, measured light, and light calculated using the Birks model. (Bottom) The ratio of the calculated light to the measured light (L c /L m ) for the five energies.

Proffered Papers: PHY 2: Treatment planning in particle therapy

OC-0084 A novel method to estimate mean excitation energies and their uncertainties for particle therapy E. Baer 1,2 , P. Andreo 3 , A. Lalonde 4 , G. Royle 1 , H. Bouchard 4 1 University College London, Medical Physics and Biomedical Engineering, London, United Kingdom 2 National Physical Laboratory, Acoustics and Ionising Radiation Team, Teddington, United Kingdom 3 Stockholm University and Karolinska University Hospital, Department of Medical Radiation Physics, Stockholm, Sweden 4 Université de Montréal, Department of Physics, Montréal, Canada Purpose or Objective Methods to precisely determine elemental compositions using for example dual-energy computed tomography exist and can be used as input data for particle therapy planning in the near future. Uncertainties arise from elemental mean excitation energies ( I -values). Currently used I -values were proposed in 1981, and no thorough uncertainty budget exists. The aim of this study is to revisit elemental I -values for clinical particle therapy planning and establish an uncertainty budget for tissue relative stopping powers (RSPs) and particle range arising from I -values and the Bragg additivity rule. Material and Methods We propose a method to optimize elemental I -values for the use in compounds, by using measured I -values of different materials (quoted in ICRU 37) and least square