Program Information
A Water-Content Based Formalism to Mitigate Stopping Power Ratio Uncertainties Caused by Mean Excitation Energy Uncertainties in Hadron Therapy
V De Smet1*, R Labarbe2 , F Vander Stappen2 , B Macq1 , E Sterpin3,4 , (1) Universite Catholique de Louvain, ICTEAM-ELEN, Louvain-la-Neuve, Belgium, (2) Ion Beam Applications (IBA), Louvain-la-Neuve, Belgium, (3) Universite Catholique de Louvain,Institut de Recherche experimentale et clinique, Center for Molecular Imaging, Radiotherapy and Oncology, Brussels, Belgium, (4) KU Leuven, Department of Oncology, Laboratory of Experimental Radiotherapy, Leuven, Belgium
Presentations
MO-L-GePD-TT-4 (Monday, July 31, 2017) 1:15 PM - 1:45 PM Room: Therapy ePoster Theater
Purpose: In proton therapy planning, the Stopping Power Ratios (SPR) calculated in the stoichiometric CT calibration are affected by, amongst others, uncertainties on the mean excitation energies (I-values) of human tissues and water. Paganetti (2012) estimated this contribution to the SPR uncertainty to about 1.5% at 1 standard deviation. This study provides an alternative methodology to evaluate this uncertainty and aims at showing, through a detailed tissue-specific analysis, that this contribution has been globally overestimated.
Methods: Since human tissues contain water, a correlation exists between the I-values of tissues and water. Our formalism considers this by expressing the I-value of the tissue as a function of the water weight fraction and the I-value of water, while applying Bragg’s additivity rule only to the non-aqueous mixture of the tissue. For 23 tissue compositions from ICRU Report 44, the SPR uncertainty was estimated by simulating Gaussian distributions, based on ICRU data, for the I-values of water and the non-aqueous mixture, as well as for the water weight fraction.
Results: The relative standard deviation of the SPR, estimated at 150 MeV, is in the range of 0.1%-0.3% for soft tissues with an average water weight percentage of at least 60%. For tissues with a low water content (e.g. adipose and bones), this uncertainty is in the range of 0.5%-0.8%. Although the uncertainties tend to increase for decreasing proton energies, they do not vary strongly in the therapeutic range, except below ~10 MeV (near the end of the proton trajectories).
Conclusion: Uncertainties on the I-values of human tissues and water appear to have a significantly lower impact on the SPR uncertainty than initially expected. In the future, this may provide a rationale for using smaller margins on the target volume, provided that all other SPR uncertainty components are correctly estimated too.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by a research grant from IBA S.A. (Belgium).
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