Stopping power measurement and calculation for proton theraphy by dual energy computed tomography

The accurate knowledge of effective atomic number (Zeff) and electron density (e) of target tissues are needed for calculations of proton's energy losses in unit length (stopping power) and the path they take in the target material (range). Since dual-energy computed tomography (DECT) provides two different energies, it gives more information for precise calculation of Zeff and e. However, influence of CT imaging artefacts due to the beam hardening effect, scattering, noise, patient movement, etc. increases uncertainty of results. In this study, phantom with 9 different tissue equivalent material on Somatom Definition As 20 Siemens DECT was scanned for 5 different energies by using 80 kVp and 140 kVp protocols together and Hounsfield Unit (HU) values obtained. The main reason for using two X-ray energies was to reduce the effects of beam hardening and raw-data based iterative reconstruction (SAFIRE) algorithm has been used to reduce standard deviation (SD). By Using in SP formula the calculated Zeff values for the 10 different energy pairs obtained, the results were compared to SRIM program, NIST database and the results of the experimental range obtained for water, found that the 70-120 keV X-ray beam for water gave the closest results.