Monte Carlo based voxel phantoms for in vivo internal dosimetry

Full text: Accurate assessment of individual doses due to internal exposure can be obtained when distribution and content of the incorporated radionuclides can be determined by in vivo measurement of the x rays or gamma radiation emitted from the body. Relative calibration of the detection facility (a whole body counter) requires the use of specific anthropomorphic phantoms to relate the measured count rate to the internal radionuclide body burden for a given counting geometry. The purpose of this communication is to describe the computer programs developed to automatically generate the input file for MCNP Monte Carlo code starting from any segmented human anatomy data and its application to the calibration procedures for in vivo internal dosimetry. The software is based on the ability of MCNP to represent complex three-dimensional structures as lattices of individual cubic cells. The detectors are also simulated in detail according to the technical information supplied by the manufacturer. The method has been initially applied to the determination of 241Am in bone by measurement in skull and knee although it can be easily generalized to other radionuclides and organs. For the simulation of in vivo measurement, MCNP voxel models of a real human head and knee have been built based on the tomographic phantoms VOXELMAN (Yale University, USA) and NORMAN (NRPB, UK). The results have been compared with those obtained by experimental measurements with physical phantoms (Cohen and Spitz phantoms, respectively) carried out at the whole body counting facility of CIEMAT (consisting of four low energy germanium detectors Canberra LE Ge) in order to verify the accuracy of the procedure. Different relative positions source-detector were tried to look for the best counting geometry and to estimate the efficiency uncertainty. The method and the software can also be applied to model any complex or irregular geometry and its application is not limited to calibration application. Some additional application will also be discussed. (author)