Dual-energy CT spectra optimization for proton treatment planning

International audienceThe purpose of this study was to determine the optimal dual-energy spectra for the treatment planning of proton therapy. We have evaluated the effect of various voltages and tin filtration combinations on the relative electron density (RED) map accuracy and precision. The RED is directly related to the stopping-power (SP) map and thus to the accuracy of the proton range estimation. An acquisition setup representing a medium-size body irradiation was evaluated. For all spectra combinations, virtual CT projections of the Gammex 467 tissue characterization phantom were simulated with realistic energy-integrating detector response model. Two situations were simulated: an ideal case without noise (infinite dose) and a realistic situation with a Poisson noise corresponding to a 20 mGy central dose. To derive the RED maps from dual-energy imaging, the projection-based basis material decomposition method proposed by Alvarez and Macovski (1976) was implemented. It was observed that the energy separation between the incident spectra had little influence on the RED accuracy but a strong influence on the precision. Different optimal ranges of low and high energy tube voltages and additional tin thicknesses that maximize the overall accuracy and the precision of RED maps were found. However, when studying each phantom material separately, a large variability of the optimal spectra was observed. An emphasis on the materials present in the anatomical region of interest must be made during the optimization process of the dual-energy spectra