DESCRIPTION This report presents the result of the work carried out by the Work Package 6 of the CAThyMARA project. This work focused on the direct assessment of the thyroid dose or committed effective dose from measurements. As a result dose per content functions are derived for different ages (new born, 1,5,10,15-years old children and adult) and for five radio-iodine isotopes (131-135 I) and 132 Te. Thyroid dose to the foetus is also obtained from measurement of the mother. If short-lived radioiodine isotopes cannot be measured correction factors to 131 I measurement are given. The interpretation of measurements in case of stable iodine prophylaxis is also addressed. This report also discusses the parameters influencing thyroid dose assessment. Finally a review of existing software for internal dose assessment is given and a demonstration software for dose assessment in case of nuclear emergency is presented. Approval WP5 leader Lara Struelens: 30/05/2017 OPERRA coordinator Jean-René Jourdain: 31/05/2017 2 1. PREFACE This report presents the methods and the results regarding the direct dose assessment from measurement of radio-iodine isotopes in the thyroid. Several cases are considered: from the newborn to the adult, the foetus and public members of the public who took stable iodine. This report also discusses the sources of uncertainties related to dose assessment, presents a review of existing software for dose estimation in case of emergency and finally presents a demonstration software dedicated to both professionals and citizens carrying out their own measurements. It is necessary, prior to any other text, to mention that materials presented here are based on some assumptions and a priori set boundaries. The considered scenarios cover serious accidents on light water nuclear reactors (PWR/BWR) in which population is subject of single acute intake. Dose assessment is performed during early phase after the accident, thus no intake by (chronical) ingestion is covered. Intake by ingestion was not covered in this project for two reasons: firstly because of the time limitation to carry-out the work, secondly because it can be reasonably assumed that food ban will be correctly implemented and followed after a reactor accident, at least for a vast majority of the population. One have to keep in mind, that the main purpose of the dose assessment system described in the CAThyMARA guidelines and here is the classification of subjects to those who need medical attention or follow-up and to those who not. Dose reconstruction for epidemiology is not considered as a primary goal, since is usually done when more accurate data are available. Thus robustness and easy use of the dose evaluation method, particularly in situations without detail information, was prioritised over high precision assessment. The system stands on old, however well-established, models related to ICRP 60 general recommendation, i.e. it does not consider dosimetric models and feature from newer ICRP documents (e.g. ICRP 103, 100, 130). The main reason for such an approach is that the updated ICRP dosimetric models were not yet fully available and implemented at the time of this project. However the use of the older system does not necessarily limit applicability since it is in general conservative. Moreover introduced systems may be easily adopted to the new one when needed. Simplistic approaches, such as one dose per measurement function for rather large spectrum of intake parameters, is used when inaccuracy due to aggregation is smaller than uncertainty of the input parameters. Other arguments for this simple approach is that it is less prone to evaluation error (mismatching) and may be used by a less skilled personnel. While guidelines is written in prescriptive form, this report is more of scientific nature. It shows particular issues in detailed way and tries to indicate limitation of the used approach in order to explain the methodology used in the guidelines. It may be read independently however it is recommended to read the guidelines prior to this report.
Research on iodine metabolism was reviewed with special reference to short-term processes, pregnancy, and lactation. A detailed discussion of the new physiologically-oriented biokinetic model and a model parameterization procedure are given. Predictions of the new model are applicable for the analysis of in vivo and in vitro clinical data gained using iodine or radioactive tracers, as well as for the assessment of radiation exposure doses to mothers and offspring. The model can be used to simulate biokinetic processes in the human body, for prediction of biokinetic parameters such as the time course of thyroid uptake (both for mother and fetus), the rate of exertion with urine, and the level of iodine secretion with saliva and breastmilk. The structure of the model is applicable in a wide range of stable iodine in the diet, both for pregnant and non-pregnant persons.
The prophylactic administration of 100 to 200 mg of potassium iodide in anticipation of radioactive iodine exposure will largely prevent uptake by the thyroid gland, thereby reducing the irradiation dose delivered by more than 98%. The same amount given at intervals after131I absorption is progressively less effective, but still reduces uptake to less than half after a delay of three hours. The suppressive effect of one dose is of short duration and daily readministration of the agent is required for prolonged protection. The extent of uptake blockade may be estimated by measuring serum inorganic iodide; concentrations of greater than 10μg /100 cc correlate with marked uptake arrest. Toxicity is negligible for a single ingestion, as is required in this temporary measure for the reduction of the immediate thyroid irradiation hazard, but the drug must be avoided by those allergic to it.
Internal dosimetry support system implements a full set of the ICRP internal dosimetry models, including the new respiratory tract model: it is a 32-bit program that operates under Windows 95. It enables the calculation of the time course of activity, the dose rate, the dynamics of dose accumulation and the committed dose in any tissue of the body; the daily urinary and faecal excretion rates also are available. The results may be accessed for the analysis and interpretation in graphical or numerical form and may be stored for future retrieval. Additional capabilities provide the means for stochastic simulation. IDSS allows analysis of the standard and non-standard exposure conditions and assists in assessment of the dose reliability. The main distinguishing features of IDSS are: processing of an arbitrary time course of the intake function, extended possibilities of data analysis, an orientation to a wide spectrum of research problems, an object-oriented logical structure and a modularity of the code. IDSS assists in the assessment of doses under normal or accidental conditions, both for prospective and retrospective dosimetry problems.
In a breach-of-containment nuclear reactor accident, the near-field exposure is primarily through inhalation of radioiodine. Thyroid blockade by oral potassium iodide (KI) is a practical and effective protective measure for the general public in such an emergency. The retention functions incorporating the thyroid blocking effects by KI have been derived using a standard three-compartment model of iodine metabolism. This allows more accurate estimation of the thyroid dose by calculating the blocking factor.
The purposes of this study were to investigate the influence of the consequences of the lack of primary bioassay information and to elaborate approaches which could improve the reliability of dose assessments. The aggregated time-dependent functions 'dose per unit organ (excretion) content' z(t) have been proposed in this study as a convenient and reliable tool for bioassay. The analysis of the variation of z with changes of AMAD has demonstrated the existence of areas of the relative invariance of z, which permits the selection of one (reference) function z for the whole area of stability. Within the framework of such an approach an arbitrary set of bioassay data can be approximated by the linear combination F(t) = sum(i) E(i)/z(t - tau(i)), where F(t) = function of time t, which approximates the observed bioassay time trend; tau(i) = time shift of the acute intake i; E(i) = effective dose, associated with the acute intake i (the two last parameters are results of the approximation procedure).
The ‘Individual Monitoring for Internal Exposure’ (IMIE) computer codes are the family of interactive tools for interpretation
of the bioassay data and individual dose assessments. During 10 y of the IMIE evolution, the extensive experience in the IMIE
application has been accumulated and the underlying methodology has undergone essential improvements. The main distinguished
feature of the IMIE ideology is the automated numerical analysis of all examined exposure scenarios with the succeeding interactive
identification of events of intake(s) and assessment of associated doses. The numerical deconvolution algorithms and the massive
library of tabulated ‘bioassay/dose response functions’ are employed for the processing of an arbitrary pattern of intake
and complex exposure conditions. This article discusses details implemented in the IMIE algorithms and examples of IMIE application.
Radioiodine biokinetics in the mother and fetus
Berkovski, V., 1999. Radioiodine biokinetics in the mother and fetus. Part 1. Pregnant Women.
Medical Effectiveness of Iodine Prophylaxis in a Nuclear Reactor Emergency Situation and Overview of European Practices
European Commission, 2010. Medical Effectiveness of Iodine Prophylaxis in a Nuclear Reactor
Emergency Situation and Overview of European Practices. Radiation Protection No. 165.