Study of the influence of biokinetics of radionuclides on the calibration coefficient of in vivo counting by Monte Carlo simulation
Congress title :52nd Annual Meeting of the Health Physics Society
Congress town :Portland
Congress date :08/07/2007
The software package OEDIPE (French acronym for 'tool for personalized dose assessment') has been developed to model internal contamination, simulate in vivo counting, and perform dose calculation. OEDIPE uses voxel phantoms and the particle transport code Monte-Carlo N Particle (MCNP and MCNPX). As the distribution of the contamination changes in time according to the biokinetics of the radionuclide, OEDIPE was changed to model heterogeneous sources consistent with the distribution of the radionuclides in the body tissues at any time as estimated by the Dose CALculation (DCAL) code. After inhalation of moderately soluble, or insoluble material, the lung activity is typically estimated with a measurement device calibrated with a physical phantom including a source located in the lungs only. However, the photons detected during the in vivo measurement come not only from the organ of interest but also from neighbouring organs. Besides, the actual values of the biokinetic parameters are often assumed rather than known. An application of OEDIPE to a case of acute inhalation of Am-241 modeled in the Zubal voxel phantom is presented to show the influence of a heterogenous distribution of activity and the influence of biokinetic parameters (median diameter and absorption type of the aerosol) on the calibration coefficient of lung counting at early times after inhalation and liver and bone counting at later times. The coefficients calculated according to the predictions of the biokinetic model are compared with those obtained from a source exclusively located in either the lungs, knee or liver. Significant differences are observed as a function of time that lead to increasing differences in the estimate of activity in the three organs. These differences represent an estimation of the error made when a typical calibration coefficient is used for the assessment of the activity. Furthermore, the propagation of the uncertainty on the biokinetic parameters to the uncertainty on the measured activity are discussed. Finally, this study provides an indication of which counting geometries should be used at different times after Am-241 inhalation and a table of time-dependant correction factors that can be applied to the calibration coefficient obtained with a physical phantom.