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Technische Universität München Physik-Department Study of The Technische Universität München Physik-Department Study of the biokinetics of zirconium isotopes in humans and its relevance to internal dosimetry Matthias Greiter Vollständiger Abdruck der von der Fakultät für Physik der Technischen Universität München zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften genehmigten Dissertation. Vorsitzender: Univ.-Prof. Dr. J. L. van Hemmen Prüfer der Dissertation: 1. Hon.-Prof. Dr. H.G. Paretzke 2. Univ.-Prof. Dr. F. von Feilitzsch Die Dissertation wurde am 27.09.2007 bei der Technischen Universität München eingereicht und durch die Fakultät für Physik am 22.04.2008 angenommen. i Author contact information: Matthias Greiter Institute of Radiation Protection Helmholtz Zentrum München German Research Center for Environmental Health (GmbH) Ingolstaedter Landstrasse 1 85764 Neuherberg Germany E-mail: [email protected] ii Table of contents Abstract ...................................................................................................................................... 1 List of acronyms, symbols and abbreviations ............................................................................ 2 1 Aim of the study......................................................................................................... 3 1.1 Natural occurrence of zirconium compounds ............................................................ 3 1.2 Applications of stable zirconium................................................................................ 5 1.3 Applications of unstable isotopes of zirconium ......................................................... 5 1.4 Current biokinetic model of the International Commission on Radiological Protection (ICRP)....................................................................................................... 7 2 Investigations with stable isotope tracers................................................................. 11 2.1 Double isotope technique and obtainable data......................................................... 11 2.2 Preparation of tracer solutions.................................................................................. 12 2.3 Experimental scheme ............................................................................................... 14 3 Measurement methods ............................................................................................. 16 3.1 Requirements............................................................................................................ 16 3.2 Mass spectrometric methods .................................................................................... 16 3.2.1 Sample preparation for thermal ionisation mass spectrometry (TIMS)........... 21 3.2.2 Description of the TIMS instrument and measurement ................................... 24 3.2.3 Tracer concentration calculation with the isotope dilution technique.............. 26 3.3 Activation analysis................................................................................................... 28 3.3.1 Proton nuclear activation (PNA) sample preparation ...................................... 31 3.3.2 PNA instrumental setup and experimental conditions ..................................... 32 3.3.3 PNA tracer concentration calculation .............................................................. 34 4 Errors and uncertainty.............................................................................................. 38 4.1 Tracers and samples ................................................................................................. 40 4.2 TIMS ........................................................................................................................ 41 4.3 PNA.......................................................................................................................... 42 5 Model development.................................................................................................. 44 5.1 Fractional absorption................................................................................................ 44 5.1.1 Double tracer technique ................................................................................... 45 5.1.2 Convolution integral technique........................................................................ 46 5.2 First-order kinetic compartment models .................................................................. 47 5.2.1 Theoretical a priori identifiability .................................................................... 48 5.2.2 Model development process............................................................................. 49 5.3 Dosimetry................................................................................................................. 53 6 Results and discussion............................................................................................. 57 6.1 Investigations ........................................................................................................... 57 6.2 Sample measurements and method comparison....................................................... 58 6.2.1 TIMS and PNA detection limits....................................................................... 58 6.2.2 Combined uncertainty...................................................................................... 61 iii 6.2.3 Correlation of TIMS and PNA results.............................................................. 62 6.2.4 Measured data.................................................................................................. 65 6.3 Model development.................................................................................................. 69 6.3.1 Fractional absorption without compartment models........................................ 69 6.3.2 Tracer kinetics and compartment model structure ........................................... 72 6.3.3 Proposed new model ........................................................................................ 81 6.4 Dosimetry................................................................................................................. 89 6.4.1 Reproducibility of ICRP ingestion dose coefficients....................................... 89 6.4.2 Influence of daughter radionuclide modelling on dose coefficients ................ 90 6.4.3 Gender differences due to SEE values ............................................................. 91 6.4.4 Differences due to alimentary tract models...................................................... 92 6.4.5 Dose coefficients of 95Zr for the proposed new zirconium biokinetic model .. 93 6.4.6 Gender differences resulting from the use of the HAT model......................... 93 6.4.7 Effect of fractional absorption on ingestion dose coefficients......................... 95 6.4.8 Contribution of individual target tissues to effective dose............................... 96 6.4.9 Summary of dosimetry................................................................................... 100 7 Conclusions............................................................................................................ 101 Acknowledgements ................................................................................................................ 103 Bibliography........................................................................................................................... 105 Annex ..........................................................................................................................................I A. TIMS method development.........................................................................................I B. Algorithm for the calculation of absorption rates ....................................................VI iv Abstract Internal radiation dosimetry relies on biokinetic models linking exposure and dose, since internal dose cannot be measured readily in exposed persons. The structure and parameters of these models are usually estimated from animal and, if available, human study data. The purpose of the present work is to extend the limited knowledge about the element zirconium. Data on zirconium metabolism, especially fractional absorption, retention in the body and excretion, are obtained directly from humans by the use of enriched stable zirconium isotopes as tracers. Thermal ionisation mass spectrometry (TIMS) and proton nuclear activation (PNA) are optimised for the measurement of nanogram amounts of tracer zirconium in biological samples. Both methods are evaluated with respect to detection limit, uncertainty and practical applicability. Blood plasma and urinary data from nine double tracer studies with up to 100 d duration serve as input for developing a new first order kinetic compartment model. The new model is based on the current zirconium model proposed by the International Commission on Radiological Protection (ICRP). Ingestion dose coefficients for 95Zr are presented for the new model. Depending on gender and the chemical form of the ingested zirconium, the resulting dose coefficients differ from the current ICRP values by – 87 % to + 64 %, with a median deviation of – 3.3 %. Effective doses vary between 73 % and 142 % from the ICRP value. The tissue-weighted contributions of individual dose coefficients to the effective dose are dominated by the colon (63 – 86 %), followed by gonads (22 % in women, 2 – 3 % in men), stomach, red bone marrow, urinary bladder
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