Modeling of body tissues for Monte Carlo simulation of radiotherapy planned with conventional x-ray CT

For accurate Monte Carlo simulation of radiotherapy, CT number of patient image is converted to mass density and elemental composition using functions or tables that are calibrated specifically for the relevant x-ray CT system. This study proposes an alternative and more practical approach of two-step conversion. Using the latest compilation of standard tissues for the reference adult male and female phantoms, we defined the representative tissues per density region by averaging mass density and composition for the regional tissues. We then formulated polyline relations between mass density and electron density, stopping-power ratio and elemental densities by connecting those of the representative tissues and compared with the preceding formulations in the literature. We also formulated a procedure of stoichiometric calibration for the first-step conversion and demonstrated the two-step CT-number conversion for an ideal x-ray CT model of 80-keV photons. For the standard tissues, there was generally high correlation between mass density and the other densities except of C and O for light spongiosa tissues occupying 1% of body mass. The polylines fitted to the dominant tissues and were generally consistent with the preceding formulations. The two-step conversion procedure was demonstrated to be practical and will potentially facilitate Monte Carlo simulation for treatment planning and for retrospective analysis of treatment plans with little impact on the management of planing CT systems.55th Annual Conference of the Particle Therapy Co-operative Grou