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

Abstract

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 D14 and D5 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 D14 was found to be slightly but systematically higher than D5. Converting dose to medium to dose to water (D14 to D14wat and D5 to D5wat) resulted in larger local differences as D5wat became up to 10% higher than D14wat. In conclusion, multiple bone bins need to be introduced when Monte Carlo (MC) calculations of patient dose distributions are converted to dose to water.