Correlation between CT numbers and tissue parameters needed for Monte Carlo simulations of clinical dose distributions

We describe a new method to convert CT numbers into mass density and elemen tal weights of tissues required as input for dose calculations with Monte C arlo codes such as EGS4. As a first step, we calculate the CT numbers for 7 1 human tissues. To reduce the effort for the necessary fits of the CT numb ers to mass density and elemental weights, we establish four sections on th e CT number scale, each confined by selected tissues. Within each section, the mass density and elemental weights of the selected tissues are interpol ated. For this purpose, functional relationships between the CT number and each of the tissue parameters, valid for media which are composed of only t wo components in varying proportions, are derived. Compared with convention al data fits, no loss of accuracy is accepted when using the interpolation functions. Assuming plausible values for the deviations of calculated and m easured CT numbers, the mass density can be determined with an accuracy bet ter than 0.04 g cm(-3). The weights of phosphorus and calcium can be determ ined with maximum uncertainties of 1 or 2.3 percentage points (pp) respecti vely. Similar values can be achieved for hydrogen (0.8 pp) and nitrogen (3 pp). For carbon and oxygen weights, errors up to 14 pp can occur. The influ ence of the elemental weights on the results of Monte Carlo dose calculatio ns is investigated and discussed.