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La Recherchev2

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Uncertainties in the determination of the internal dose to the thyroid.


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Workshop on Internal Dosimetry of Radionuclides - Occupational, Public and Medical Exposure - 9-12 September 2002 - New College, Oxford, United Kingdom JR. Jourdain, I. Aubineau-Lanièce, L. de Carlan, and D. Franck IRSN (Institut de Radioprotection et de Sûreté Nucléaire) Département de Protection de la santé de l’Homme et de Dosimétrie - Service de dosimétrie BP17 - 92262 Fontenay-aux-Roses Cedex - France

Résumé

Estimates of intake and dose to thyroid from radioactive iodine are typically derived from in vivo measurements and from mathematical models for a reference individual with specified anatomical, physiological and metabolic characteristics. So, the starting point of the process is the result of a particular monitoring measurement (activity in the thyroid), and the end point is a calculated value (organ dose). Nevertheless, steps between these two points will introduce some uncertainties that must be identified and quantified in order to estimate the magnitude of their effects on assessed dose. As a result of the knowledge of these uncertainties, the conditions of the measurement and the choice of the parameters involved in the dose calculation could be optimised in order to get the “best estimate” of the contamination. The aim of this paper is to investigate the major sources of uncertainties in doses to the thyroid as a result of the monitoring programmes. The complete process of dose assessment will be considered, including measurements, data analysis, biokinetic modelling, model parameters selection and evaluation of dose coefficients. Data have been compiled from the published literature for each parameter involved in the determination of the thyroid intake from ingested iodine-131 (choice of detector, detector positioning, neck-to-detector distance, background counting rate, accuracy of initial calibration standard, thyroid depth) and in the calculation of estimates of dose to the human thyroid gland (mass of the thyroid, fraction of ingested iodine absorbed by the thyroid, biological half-time of residence). The importance of these uncertainties in dose estimates will be discussed, including potential implications for radiation protection.