Optimization of the stopping-power-ratio to Hounsfield-value calibration curve in proton and heavy ion therapy

For CT-based dose calculation in ion therapy a link between the attenuation coefficients of photons and the stopping-power of particles has to be provided. There are two commonly known approaches to establish such a calibration curve, the stoichiometric calibration and direct measurements with tissue substitutes or animal samples. Both methods were investigated and compared. As input for the stoichiometric calibration the data from ICRP-report 23 were compared to newly available data from ICRP-report 110. By employing the newer data no relevant difference could be observed. The differences between the two acquisition methods (direct measurement and stoichiometric calibration) were systematically analyzed and quantified. The most relevant change was caused by the exchange of carbon and oxygen content in the substitutes in comparison to the data of the ICRP-reports and results in a general overshoot of the Bragg peak. The consequence of the differences between the calibration curves was investigated with treatment planning studies and iso-range surfaces. Range differences up to 6 mm in treatment plans of the head were observed. Additionally two improvements are suggested which increase the accuracy of the calibration curve.