Application du calcul numérique utilisant les fantômes voxelisés et l'expérimentation animale pour l'étude de l'influence de la distribution d'actinides dans les poumons sur les facteurs d'étalonnages en mesure in vivo pulmonaire

Titre du congrès :IRPA 2006 2nd European IRPA Congress on Radiation Protection Ville du congrès :Paris Date du congrès :15/05/2006

Calibration of lung counting system dedicated to the assessment of the retention of actinides in the lungs remains delicate due to large uncertainties in calibration factors. Among them, the detector positioning, the chest wall thickness assessment and its composition (muscle/fat) and the distribution of the contamination are the main parameters influencing the detectors response. In order to improve these uncertainties, a numerical approach based on the application of voxel phantoms (numerical phantoms based on tomographic images, CT or MRI) associated to a Monte-Carlo code (namely MCNP) was developed. It led to the development of a dedicated tool, called OEDIPE, that allows to easily handle realistic voxel phantoms for the simulation of in vivo measurement (or dose calculation, application that will not be presented here). The goal of the paper is to present a first approach of the study of the influence of the source distribution in the lungs on calibration factors using this numerical method. Indeed, calibration phantoms, like Livermore phantom, always consider a uniform distribution of the source in the lungs, which is probably not exactly true. Since very few data are available on the geometric pattern of distribution in the lungs, a study was initiated through collaboration with the radiotoxicology laboratory of CEA since this laboratory is specialized in biokinetic studies after inhalation based on experimentations with primates. The objective of the study is to compare the results of a real distribution of the source (obtained form animal experimentations) with the homogeneous one considered as the reference. This comparison was performed thanks to OEDIPE that can almost simulate any source distribution. The study was conducted first with the experimental approach consisting of contaminating the primate (baboon of 8 kg) and after the sacrifice (one month after the contamination) to obtain the distribution of the contamination in the lungs by gamma spectrometry after dissection of different lobes of the baboon (many other experiments were conducted for biokinetic purposes that are not described here). In the numerical approach, the first data required was the numerical phantom of the baboon. For that purpose, a CT scan was performed on a similar baboon to build the voxel phantom. After segmentation of the phantom and separation of the different lobes in the lungs, the experimental source distribution obtained was reproduced in the voxel phantom and simulation of the measurement was performed (measurement system composed of two detectors). The last step was the comparison by simulation of the results between the real source and a homogeneous one in the lungs. Preliminary results show large discrepancies between right and left lung as it was seen with the measured distribution of the contamination for energies from 17 to 60 keV. Nevertheless, by summing the contribution of both detectors, the differences are about 10 %. This first study shows the potential of the simulation associated to experimental data for the assessment of uncertainties in in vivo measurement. Further developments will be focused on the influence of biokinetic models on in vivo measurements