Estimating individual thyroid doses for a case-control study of childhood thyroid cancer in Bryansk Oblast, Russia

Following the Chernobyl accident, radioactive fission products, including (131)I and (137)Cs, were deposited in Bryansk Oblast in Russia. Intakes of radioiodines, mainly (131)I in milk, were the principal sources of radiation doses to thyroids of residents of the contaminated areas, but those radionuclides decayed before detailed contamination surveys could be performed. As a result, (137)Cs deposition density is the primary measure of the contamination due to the accident and there are relatively few measurements of the ratio of (131)I to (137)Cs in vegetation or soil samples from this area. Although many measurements of radiation emitted from the necks of residents were performed and used to estimate thyroidal (131)I activities and thyroid doses, such data are not available for all subjects. The semi-empirical model was selected to provide a dose calculation method to be applied uniformly to cases and controls in the study. The model was developed using dose estimates from direct measurements of (131)I in adult thyroids, and relates settlement average thyroid doses to (137)Cs contamination levels and ratios of (131)I to (137)Cs. This model is useful for areas where thyroid monitoring was not performed and can be used to estimate doses to exposed individuals. For application to children in this study, adjustment factors are used to address differences in age-dependent intake rates and thyroid dosimetry. Other individual dietary factors and sources (private/public) of milk consumed are reflected in the dose estimates. Countermeasures that reduced thyroid dose, such as cessation of milk consumption and intake of stable iodine, are also considered for each subject. The necessary personal information of subjects was obtained by interview, most frequently of their mothers, using a questionnaire developed for the study. Uncertainties in thyroid dose, estimated using Monte Carlo techniques, are presented for reference conditions. Thyroid dose estimates for individual children made using the semi-empirical model and questionnaire data compare reasonably well with dose estimates made for 19 children whose thyroid burdens of (131)I were measured from May to June 1986.     

Uncertainty in particulate deposition for 1 microm AMAD particles in an adult lung model

The ICRP 66 lung model may be used to determine dose estimates for members of the public via the inhalation pathway. A significant source of uncertainty in internal dosimetric modelling is due to particulate deposition in regions of the respiratory tract. Uncertainties in estimates of particulate deposition are present because model input parameters have their own inherent variability. These sources of uncertainty need to be examined in an effort to understand better model processes and to estimate better the doses received by individuals exposed through the inhalation pathway. An improved understanding of the uncertainty in particulate deposition will further guide research efforts and improve our ability to quantify internal dose estimates. The ICRP 66 lung deposition model is most sensitive to breathing rate when 1 microm AMAD particles are inhaled by members of the public. Uncertainties in deposition fractions are shown to span an order of magnitude with their distributions varying by gender for a particular lung region. The largest fractional deposition occurs in the deep lung alveolar and extrathoracic regions.     

Age-dependent potassium iodide effect on the thyroid irradiation by 131I and 133I in the nuclear emergency

The initial near-field exposure is primarily through inhalation in a nuclear emergency and the dominant contribution to the effective inhalation dose comes from radioiodine. Thyroid blockade by oral potassium iodide (KI) is efficient and practical for public in the nuclear emergency. Age-dependent radioprotective effect of KI on the thyroid irradiation by (131)I and (133)I has been derived using the simplified compartment model of iodine metabolism and WinSAAM program. Administration of KI within 2 h after (131)I and (133)I intake can block thyroid uptake significantly, yielding protective effect of 78.9% and 74.3%, respectively, for (131)I and (133)I for adults. The mean absorbed doses decrease with age, while protective effects of KI are similar for all age groups.     

Uncertainties in thyroid dose reconstruction after Chernobyl

The most significant impact of the Chernobyl accident of 26 April 1986 is the increased incidence of thyroid cancer among Belarusians, Russians and Ukrainians who were exposed as children to radioiodines in fallout resulting from atmospheric releases. The US National Cancer Institute (NCI), in cooperation with the ministries of health of Belarus and Ukraine, is involved in epidemiological studies of thyroid diseases related to the accident. Individual thyroid doses, as well as uncertainties, have been estimated for the members of the cohort studies (approximately 13,000 Ukrainians and 12,000 Belarusians). The cohort subjects, who were selected from the large group of children whose thyroids were monitored for gamma radiation within a few weeks of the accident, provided personal information on their residence history and dietary habits during interviews. Thyroid dose estimates range from 1 mGy to more than 20 Gy. The uncertainties are found to be approximately log-normally distributed, with geometric standard deviations ranging from 1.6 to 5.0. The medians of the geometric standard deviations are 1.7 for the Ukrainian subjects and 2.1 for the Belarusian subjects. The major sources of uncertainty in the thyroid dose estimates are found to be those related to the thyroid mass of the subject and to the estimation of the thyroidal content of 131I at the time of thyroid monitoring.     

Contributions of short-lived radioiodines to thyroid doses received by evacuees from the Chernobyl area estimated using early in vivo activity measurements

A series of in vivo gamma spectrometric measurements of 65 people evacuated from Pripyat 1.5 days after the Chernobyl Nuclear Power Plant Unit 4 explosion was performed in St Petersburg, Russia, as early as 30 April 1986. The historical spectra and interviews were recently processed and the results used for thyroid dose estimation. Activities of 131I in thyroid and 132Te in lungs were determined easily; for estimation of 132I and 133I activities in thyroid, sophisticated methods of spectral processing were developed. According to thyroid measurement data, the mean ratio of 133I/131I activities (at the time of the accident) inhaled by residents of Pripyat was 2.0. The mean ratio of thyroid dose from 133I inhalation to that caused by 131I amounts to 0.3, which confirms the accuracy of dose estimates based on the evolution of the Chernobyl accident. The mean ratio of 132I activity in thyroid to that of 132Te in lungs was assessed from the human measurement data to be 0.2, which is in reasonable agreement with the metabolic properties of these radionuclides. The mean ratio of thyroid dose from 132I originating from 132Te deposited in lungs to the dose caused by 131I was 0.13 +/- 0.02 for Pripyat residents who did not take KI pills and 0.9 +/- 0.1 for persons who took KI pills. Thus, the contribution of short-lived radioiodines to total thyroid dose of Pripyat residents, which was on average 30% for persons who did not use stable iodine prophylaxis, and about 50% for persons who took KI pills on 26-27 April, should be accounted for in the assessment of thyroid health effects.     

Thyroid dosimetry in the western trace of the Chernobyl accident plume

According to World Health Organization guidelines (WHO/SDE/PHE/99.6), the reference level for consideration in stable iodine prophylaxis is based on the inhalation exposure pathway. In the western trace of the Chernobyl accident, the measurement of airborne (131)I fractions (aerosol-associated, gaseous reactive and gaseous organic) indicates that airborne gaseous reactive and, especially, organic (131)I fractions were the major contributors to thyroid exposure due to inhalation. The contribution of inhaled short-lived radioiodines was negligible. To attain more precise thyroid exposure evaluation, (131)I dose factors were determined as a function of age and prevalence of stable iodine deficiency. The results demonstrate that children with a stable iodine deficiency experienced at least two times higher thyroid doses than did children with a dietary iodine sufficiency. The results of these investigations demonstrate that in thyroid dosimetry it is important to know the stable iodine status as well as to have a standardised method for airborne radioiodine measurements, especially for consideration of stable iodine prophylaxis based on the inhalation exposure pathway.     

Thin layer alpha-Al2O3:C beta dosemeters for the assessment of current dose rate in teeth due to 90Sr intake,and comparison with electron paramagnetic resonance dosimetry

The use of thin-layer alpha-Al2O3:C thermoluminescent detectors (TLDs) for the assessment of current beta dose rate in human teeth due to 90Sr intake is investigated. The teeth used in this study were collected from members of the Techa river population who were exposed to radiation as a result of releases of the Mayak plutonium production facilities (Southern Urals-Russia) between 1949 and 1956. The beta dose rates from different parts of the tooth (enamel, crown dentine, and root) were determined by storing the detectors over the samples in a shielded environment. The cumulative dose measured by electron paramagnetic resonance (EPR) in different dental tissues is found to be proportional to current dose rate obtained from alpha-Al2O3:C thermoluminescence dosemeters. The retention of 90Sr in various parts of the teeth is discussed.     

Uncertainties in determination of 131I activity in thyroid gland

The values of the iodine (131)I uptake determined from the measurements with a NaI scintillation counter might be significantly influenced by the thyroid position in the human neck. It is shown that the ratio of the counting rate in the energy peak of 364 keV to the counting rate in the Compton scattering band can be used for the determination of the effective depth of the thyroid. The uncertainties of the standard method are discussed basing on results of calibration measurements. The investigations of 95 patients with different thyroid diseases showed that the measured iodine spectrum was considerably different from the standard for about 20% of them, indicating possible high uncertainties of the standard measurements, which assume a fixed thyroid depth of 20 mm.     

A new approach to reactor safety goals in the framework of INES

By extending a framework of the International Nuclear Event Scales (INES) widely used throughout the world, the author proposes a new concept of reactor safety goals that facilitates engineering applications, while removing some of the uncertainties often encountered in implementing safety goals. The INES criteria specified in releases are translated into doses by performing dispersion calculations for a typical site to estimate radiological consequences to the public. Quantitative health objectives that incorporate lessons learned from radiological consequences of the Chernobyl accident are deployed into master risk curves. More attention is paid to immediate noble gas releases occurring in the early phase of severe accidents, as well as the delayed release of iodine and cesium that can cause wide spread land contamination. The land contaminated with radioactive iodine around Chernobyl and the subsequent ingestion of iodine through pasture-cow-milk pathways is said to have induced many of the thyroid cancer cases. By monitoring more frequent incidents with/without release, a set of master risk curves can be used to assess the operating performance of plants in a safety space delineated by three regions (acceptable, tolerable, unacceptable). It is concluded that another Level 5 or beyond radiological consequence, especially the potential land contamination by ¹³⁷Cs would be a disaster and is totally unacceptable.     

Calibration of surface contamination monitors for the detection of iodine incorporation in the thyroid gland

In Switzerland, individuals exposed to the risk of activity intake are required to perform regular monitoring. Monitoring consists in a screening measurement and is meant to be performed using commonly available laboratory instruments. More particularly, iodine intake is measured using a surface contamination monitor. The goal of the present paper is to report the calibration method developed for thyroid screening instruments. It consists of measuring the instrument response to a known activity located in the thyroid gland of a standard neck phantom. One issue of this procedure remains that the iodine radioisotopes have a short half-life. Therefore, the adequacy and limitations to simulate the short-lived radionuclides with so-called mock radionuclides of longer half-life were also evaluated. In light of the results, it has been decided to use only the appropriate iodine sources to perform the calibration.     

Influence of radionuclides distributed in the whole body on the thyroid dose estimates obtained from direct thyroid measurements made in Belarus after the Chernobyl accident

Thyroid dose reconstruction is most accurate when using the results of direct thyroid measurements, in which gamma radiation emitted by radionuclides present in the thyroid and in the remainder of the body is recorded by means of a radiation detector positioned against the neck. A large number of such measurements were performed in Belarus in May-June 1986. Owing to the short half-life of 131I and to the intake and accumulation of caesium radioisotopes (mainly 134Cs and 137Cs) in the body, the thyroid doses derived from thyroid measurements made after the beginning of June 1986 have so far been often considered to be unreliable. To evaluate the influence of the caesium radioisotopes to the signal recorded by an instrument performing measurement of 131I activity in the thyroid, a Monte Carlo method was used to calculate the calibration factors of that instrument. These calculations were made for males of six reference ages: newborn, 1, 5, 10 and 15 years old, and adult. The calibration factors were combined with estimated time-dependent intake functions for 131I and caesium radioisotopes. The fractions of the instrument indications that were due to 131I in thyroid were thus estimated as a function of the age of the subject that was measured and of the time elapsed since the accident. Using this information when processing the thyroid measurements made in May 1986 would improve the accuracy of the thyroid dose estimates, and may make it possible to use a larger proportion of the thyroid measurements made in June 1986.     

Uncertainties on predicted concentrations of radionuclides in terrestrial foods and ingestion doses

Models are routinely used to calculate doses following routine releases and potential accidental releases of radionuclides. These models contain a number of parameters. Values for many of these are not accurately known. The primary aim of this study was to determine the level of uncertainty on predicted concentrations of radionuclides in foods, and doses to individuals consuming those foods, arising from uncertainties on the model input parameter values. A secondary objective was to identify those input parameters whose uncertainty makes a major contribution to the overall uncertainty on the predicted endpoints. The methodology adopted and results obtained are presented for the following radionuclides: 90Sr, 131I, 137Cs and 239Pu. The estimated uncertainty ratios (the ratio of the 95th to the 5th percentile) are frequently very large, often two to three orders of magnitude. The results of this study may be used to identify areas where further research could improve assessment capabilities.     

Uncertainty and sensitivity analysis of food pathway results with the MACCS reactor accident consequence model

Uncertainty and sensitivity analysis techniques based on Latin hypercube sampling, partial correlation analysis and stepwise regression analysis are used in an investigation with the MACCS model of the food pathways associated with a severe accident at a nuclear power station. The primary purpose of this study is to provide guidance on the variables to be considered in future review work to reduce the uncertainty in the important variables used in the calculation of reactor accident consequences. The effects of 87 imprecisely-known input variables on the following reactor accident consequences are studied: crop growing-season dose, crop long-term dose, milk growing-season dose, total food pathways dose, total ingestion pathways dose, total long-term pathways dose, area dependent cost, crop disposal cost, milk disposal cost, condemnation area, crop disposal area and milk disposal area. When the predicted variables are considered collectively, the following input variables were found to be the dominant contributors to uncertainty: fraction of cesium deposition on grain fields that is retained on plant surfaces and transferred directly to grain, maximum allowable ground concentrations of Cs-137 and Sr-90 for production of crops, ground concentrations of Cs-134, Cs-137 and I-131 at which the disposal of milk will be initiated due to accidents that occur during the growing season, ground concentrations of Cs-134, I-131 and Sr-90 at which the disposal of crops will be initiated due to accidents that occur during the growing season, rate of depletion of Cs-137 and Sr-90 from the root zone, transfer of Sr-90 from soil to legumes, transfer of Cs-137 from soil to pasture, transfer of cesium from animal feed to meat, and the transfer of cesium, iodine and strontium from animal feed to milk.     

Using total beta-activity measurements in milk to derive thyroid doses from Chernobyl fallout

Following the Chernobyl accident, more than 200 childhood thyroid cancer cases have been observed in Brest Oblast of Belarus in territories slightly contaminated with 137Cs, but with suspected relatively high 131I fallout. The most helpful measurements available that can be used to estimate thyroid doses for the population of Brest Oblast are the total beta-activity measurements in cow's milk performed using DP-100 device within a few weeks after the accident. The 131I concentrations in milk were derived from the total beta-activity measurements on the basis of (1) a radioecological model used to estimate the variation with time of the radionuclide composition in milk and (2) the determination of the calibration factors of the DP-100 device for the most important radionuclides present in milk. As a result, 131I concentrations in milk were reconstructed for territories with different levels of 137Cs deposition. A non-linear dependence of the 131I concentration in milk on the 137Cs deposition density was obtained; it was used to estimate the thyroid doses from the consumption of 131I-contaminated cow's milk by the population of Brest Oblast. The average individual thyroid doses have been estimated to be 0.15, 0.18, 0.12, 0.06, 0.04 and 0.03 Gy for newborn, children aged 1, 5, 10 and 15 y and adults, respectively. The collective thyroid dose for the entire population of Brest Oblast is estimated to be 64,500 man Gy, the contribution from the adult population being about one half of the total. The methodology that is described could be applied in the framework of epidemiological studies of the relationship between radiation exposure to the thyroid gland and thyroid cancer in areas where numerous total beta-activity measurements in cow's milk were performed within a few weeks after the accident.     

Some aspects of the fetal doses given in ICRP Publication 88

The International Commission on Radiological Protection (ICRP) has recently published dose coefficients (dose per unit intake, Sv Bq(-1)) for the offspring of women exposed to radionuclides during or before pregnancy. These dose estimates include in utero doses to the embryo and fetus and doses delivered postnatally to the newborn child from radionuclides retained at birth. This paper considers the effect on doses of the time of radionuclide intake and examines the proportion of dose delivered in utero and postnatally for different radionuclides. Methods used to calculate doses to the fetal skeleton are compared. For many radionuclides, doses are greatest for intakes early in pregnancy but important exceptions, for which doses are greatest for intakes later in pregnancy, are iodine isotopes and isotopes of the alkaline earth elements, including strontium. While radionuclides such as 131I deliver dose largely in utero, even for intakes late in pregnancy, others such as 239Pu deliver dose largely postnatally, even for intakes early during pregnancy. For alpha emitters deposited in the skeleton, the assumption made is of uniform distribution of the radionuclide and of target cells for leukaemia and bone cancer in utero; that is, the developing bone structure is not considered. However, for beta emitters, the bone structure was considered. Both approaches can be regarded as reasonably conservative, given uncertainties in particular in the location of the target cells and the rapid growth and remodelling of the skeleton at this stage of development.     

Quantitative analysis of the factors responsible for over or under dose of 131 I therapy patients of hyperthyroidism

Radioiodine (131 I) therapy has been in use for more than 60 y. Several protocols have been suggested and used for prescribing the activity to be administered to the patients for the treatment of hyperthyroidism; application of these protocols may result in an under or over dose of the hyperthyroid patients. The main objective of this study was to carry out quantitative analysis of the factors responsible for possible under or over dosage of the patients. In this regard, a total of 59 patients [15 diffuse goitre (DG) and 44 nodular goitre (NG) cases] were studied. In order to compare the thyroid doses calculated by using different protocols, the dosimetric approach was followed. 131 I uptakes were measured after 24 and 48 h, respectively, by giving 0.5 MBq of 131 I to each patient. Thyroid mass and effective half-life were also calculated for each patient and the variations in the thyroid doses were analysed. According to the results 28 and 54% patients were under dosed and 72 and 46% patients were over dosed with DG and NG, respectively. The protocols, which have not taken into account the thyroid mass, multi pre-therapeutic 131 I uptakes and the effective half-life of 131 I of the individual patient, showed a higher degree of deviation from the required thyroid dose. Besides these parameters, some fundamental factors such as radiosensitivity, previous exposure to thyroid drugs and duration of the disease are recommended to be incorporated, which can certainly affect the clinical out comes.     

STRRAP system—A software for hazardous materials risk assessment and safe distances calculation

This work presents a powerful computational tool (Stochastic Toxic Release Risk Assessment Package, STRRAP) useful in risk assessment and emergency planning (safe distance calculation), which allows to handle the stochastic uncertainty of atmospheric parameters, critical for risk calculation when diffusion of hazardous gases or particulate matter occur as a consequence of an emission or accidental release. In fact, the random behaviour of wind intensity, wind direction, atmospheric stability and temperature, given a time horizon, (a season or a complete year), is taken into account considering also the day or night condition.STRRAP can be used for releases or emissions from static sources (for example a stack or a fixed tank in a facility) or from transportation accidents (road, rail, maritime and pipeline transport) involving different scenarios.After a stochastic simulation based on well-known diffusion models (dense and light gases, particulate matter) is carried out, the downwind pollutant concentrations are obtained, in order to compute safe distances and/or individual and societal risks.Some study cases are analyzed to show STRRAP capabilities.     

Variability of radioiodine measurements in the thyroid

Monte Carlo simulations were carried out to study the response of a thyroid monitor for measuring intake activities of (125)I and (131)I. The aim of the study was 3-fold: to cross-validate the Monte Carlo simulation programs, to study the response of the detector using different phantoms and to study the effects of anatomical variations. Simulations were performed using the Swiss reference phantom and several voxelised phantoms. Determining the position of the thyroid is crucial for an accurate determination of radiological risks. The detector response using the Swiss reference phantom was in fairly good agreement with the response obtained using adult voxelised phantoms for (131)I, but should be revised for a better calibration for (125)I and for any measurements taken on paediatric patients.     

Simulation of the two-phase flow in a laboratory coal pulveriser

The particle-laden flow in the separator of a laboratory beater wheel mill is computationally and experimentally investigated. The main aim of the present study has been the identification of convenient modelling approaches and the assessment of the modelling predictive capability for this class of problems. For the particulate phase a Lagrangian formulation is adopted. Particle computations are carried out as post-processing, assuming different size class distributions. A modulation of the gas turbulence by the particulate phase is neglected. Different turbulent models are used. The turbulent particle dispersion is modelled by a stochastic approach. Different geometric configurations of the separator are investigated. With the results of the simulation study it became possible to predict the trends correctly.