Potentiel des nouvelles techniques d'imagerie et de calcul pour l'estimation de l'activité et de la dose pour des contaminations localisées.
L. de Carlan, I. Aubineau-Lanièce, J.R. Jourdain, B. Le Guen, N. Pierrat and D. Franck, IRPA 11, 23-28/05/2004, Madrid, Espagne.
Activity assessment is always based on a reference measurement performed using a physical phantom. Although great efforts have been made to improve theses phantoms, they represent a single average counting geometry and usually contain an uniform distribution of the radionuclide within the tissue substitute. Because in reality the deposition could be heterogeneous or even very localized in the subject, this can result in large uncertainties in the assessed activity and dose calculation. In consequence, significant assumptions have to be made to phantom-based calibration factors such as Chest Wall Thickness (CWT) and adipose content in order to obtain more realistic calibration efficiencies applicable to a given individual. Thus, it was desirable to develop a numerical method for calibrating in vivo measurement systems that is more sensitive to these types of variability (CWT, % adipose).
Our previous work has demonstrated the possibility of such a calibration using the Monte Carlo technique. Taking advantage of recent progress in image-processing codes, our research program extended such investigations to the reconstruction of numerical anthropomorphic phantoms based on personal physiological data obtained by computed tomography (CT) and magnetic resonance imaging (MRI). The new techniques, presented here, allow the direct and fast reconstruction of a realistic voxel phantom, the automatic creation of the data required by the Monte-Carlo code (MCNP) and the output data processing including a user friendly display of the results given by the code. In addition to the application dedicated to in vivo calibration purpose, the software called “Oedipe” has been extended to dose calculation allowing thus to perform the whole assessment in case of contamination.
A first application of calculations and comparison with the experimental data on a real wound contamination case are presented and discussed in this paper. This incident occurred in a materials science and metallurgy laboratory during the examination of an irradiated sample of reactor fuel assembly. The incident leads to a puncture point contamination of the person in charge of the study at the end of the 3rd finger of the left hand. The contaminant was mainly Ru-106 with its daughter Rh-106. The measurement and dosimetric results are compared with the ones obtained with conventional detection techniques and with a well-known dosimetric method.
The good agreement between measurements and calculations in the case of a punctual contamination in a simple geometry of measurement opens up the possibility to apply this method to lung contamination where the geometry of the contamination (heterogeneous deposition, complex anatomy, retention in lymph nodes…) is much more complex. First investigations in modelisation of various contamination configurations (punctual, volumic) have shown the potential of the new method presented here.