The relation between x-ray CT numbers and charged particle stopping powers and its significance for radiotherapy treatment planning

The theoretical results for CT number and relative stopping power given appear to be very reliable, except possibly when large components of high Z elements are present. This implies that the calibration curve given could be used to convert measured CT numbers to relative stopping powers for use in radiotherapy treatment planning for charged particles, such as protons, heavy ions and negative pions, particularly in regions where experimental data are not available. As materials which are tissue-equivalent for electrons and x-rays are not tissue-equivalent for pions (Lewis et al., 1982), it may be more reliable to use theoretical results for real tissues, rather than experimental results for the tissue substitutes in common use.