Conversion of CT numbers into tissue parameters for Monte Carlo dose calculations: a multi-centre study

The conversion of computed tomography ( CT) numbers into material composition and mass density data influences the accuracy of patient dose calculations in Monte Carlo treatment planning ( MCTP). The aim of our work was to develop a CT conversion scheme by performing a stoichiometric CT calibration. Fourteen dosimetrically equivalent tissue subsets ( bins), of which ten bone bins, were created. After validating the proposed CT conversion scheme on phantoms, it was compared to a conventional five bin scheme with only one bone bin. This resulted in dose distributions D-14 and D-5 for nine clinical patient cases in a European multi- centre study. The observed local relative differences in dose to medium were mostly smaller than 5%. The dose - volume histograms of both targets and organs at risk were comparable, although within bony structures D-14 was found to be slightly but systematically higher than D-5. Converting dose to medium to dose to water ( D-14 to D-14wat and D-5 to D-5wat) resulted in larger local differences as D-5wat became up to 10% higher than D-14watc. In conclusion, multiple bone bins need to be introduced when Monte Carlo ( MC) calculations of patient dose distributions are converted to dose to water.