Obtaining an unbiased estimate of intake in routine monitoring when the time of intake is unknown

Plutonium in Manhattan Project workers: Using autopsy data to evaluate organ content and dose estimates based on urine bioassay with implications for radiation epidemiology

PURPOSE

Radiation dose estimates in epidemiology typically rely on intake predictions based on urine bioassay measurements. The purpose of this article is to compare the conventional dosimetric estimates for radiation epidemiology with the estimates based on additional post-mortem tissue radiochemical analysis results.

METHODS

The comparison was performed on a unique group of 11 former Manhattan Project nuclear workers, who worked with plutonium in the 1940s, and voluntarily donated their bodies to the United States Transuranium and Uranium Registries.

RESULTS

Post-mortem organ activities were predicted using different sets of urine data and compared to measured activities. Use of urinalysis data collected during the exposure periods overestimated the systemic (liver+skeleton) deposition of 239Pu by 155±134%, while the average bias from using post-exposure urinalyses was -4±50%. Committed effective doses estimated using early urine data differed from the best estimate by, on average, 196±193%; inclusion of follow-up urine measurements in analyses decreased the mean bias to 0.6±36.3%. Cumulative absorbed doses for the liver, red marrow, bone surface, and brain were calculated for the actual commitment period.

CONCLUSION

On average, post-exposure urine bioassay results were in good agreement with post-mortem tissue analyses and were more reliable than results of urine bioassays collected during the exposure.

Bayesian Analysis of Plutonium Bioassay Data at Los Alamos National Laboratory

The main concern of operational internal dosimetry is to detect intakes and estimate doses to the worker from a series of bioassay measurements. Although several methods are available, the inverse problem of internal dosimetry-i.e., determination of time, amount, and types of intake given a set of bioassay data-is well suited to a Bayesian approach. This paper summarizes the Bayesian methodology used at Los Alamos National Laboratory to detect intakes and estimate doses from plutonium bioassay measurements. Some advantages and disadvantages of the method are also discussed. The successful application of Bayesian methods for several years at Los Alamos National Laboratory, which monitors thousands of workers annually for plutonium, indicates that the methods can be extended to other facilities.

ICRP Publication 130: Occupational Intakes of Radionuclides: Part 1

This report is the first in a series of reports replacing Publications 30 and 68 to provide revised dose coefficients for occupational intakes of radionuclides by inhalation and ingestion. The revised dose coefficients have been calculated using the Human Alimentary Tract Model (Publication 100) and a revision of the Human Respiratory Tract Model (Publication 66) that takes account of more recent data. In addition, information is provided on absorption into blood following inhalation and ingestion of different chemical forms of elements and their radioisotopes. In selected cases, it is judged that the data are sufficient to make material-specific recommendations. Revisions have been made to many of the models that describe the systemic biokinetics of radionuclides absorbed into blood, making them more physiologically realistic representations of uptake and retention in organs and tissues, and excretion. The reports in this series provide data for the interpretation of bioassay measurements as well as dose coefficients, replacing Publications 54 and 78. In assessing bioassay data such as measurements of whole-body or organ content, or urinary excretion, assumptions have to be made about the exposure scenario, including the pattern and mode of radionuclide intake, physical and chemical characteristics of the material involved, and the elapsed time between the exposure(s) and measurement. This report provides some guidance on monitoring programmes and data interpretation.

A statistical model for reconstruction of historical internal doses for uranium workers

Uranium workers are monitored for their internal doses mainly by urine measurements. During the years before the early nineties, urine samples were analysed using a fluorimetric system, and the lower limit of detection of the urine samples was relatively high, thus most of the urine measurements were recorded as below threshold. A model was developed for the reconstruction of doses to workers at uranium facilities during these years. The model is based on the assumption that the results of urine measurements are log-normally distributed and that a normalised log-normal distribution with a constant geometric standard deviation characterises the results distribution in all the uranium workplaces for workers throughout their occupational work. Therefore, the average of the urine results can be calculated from the general normalised distribution. The intake and dose can be evaluated from the reconstructed average urine concentration by assuming a constant chronic intake regime throughout the whole period of monitoring.

Internal dose assessments: uncertainty studies and update of ideas guidelines and databases within CONRAD project

The work of Task Group 5.1 (uncertainty studies and revision of IDEAS guidelines) and Task Group 5.5 (update of IDEAS databases) of the CONRAD project is described. Scattering factor (SF) values (i.e. measurement uncertainties) have been calculated for different radionuclides and types of monitoring data using real data contained in the IDEAS Internal Contamination Database. Based upon this work and other published values, default SF values are suggested. Uncertainty studies have been carried out using both a Bayesian approach as well as a frequentist (classical) approach. The IDEAS guidelines have been revised in areas relating to the evaluation of an effective AMAD, guidance is given on evaluating wound cases with the NCRP wound model and suggestions made on the number and type of measurements required for dose assessment.

Uncertainties analysis for the plutonium dosimetry model, doses-2005, using Mayak bioassay data

The Doses-2005 model is a combination of the International Commission on Radiological Protection (ICRP) models modified using data from the Mayak Production Association cohort. Surrogate doses from inhaled plutonium can be assigned to approximately 29% of the Mayak workers using their urine bioassay measurements and other history records. The purpose of this study was to quantify and qualify the uncertainties in the estimates for radiation doses calculated with the Doses-2005 model by using Monte Carlo methods and perturbation theory. The average uncertainty in the yearly dose estimates for most organs was approximately 100% regardless of the transportability classification. The relative source of the uncertainties comes from three main sources: 45% from the urine bioassay measurements, 29% from the Doses-2005 model parameters, and 26% from the reference masses for the organs. The most significant reduction in the overall dose uncertainties would result from improved methods in bioassay measurement with additional improvements generated through further model refinement. Additional uncertainties were determined for dose estimates resulting from changes in the transportability classification and the smoking toggle. A comparison was performed to determine the effect of using the model with data from either urine bioassay or autopsy data; no direct correlation could be established. Analysis of the model using autopsy data and incorporation of results from other research efforts that have utilized plutonium ICRP models could improve the Doses-2005 model and reduce the overall uncertainty in the dose estimates.

Uncertainties in doses from intakes of radionuclides assessed from monitoring measurements

The evaluation of uncertainties in doses from intakes of radionuclides is one of the most difficult problems in internal dosimetry. In this paper, the process of assessing internal doses from monitoring measurements is reviewed and the major sources of uncertainty are discussed. Methods developed independently at HPA and at IRSN for the determination of uncertainties in internal doses assessed from monitoring measurements are described. Both use a Monte Carlo simulation approach. Results are described for three illustrative examples. An alternative method developed at the Los Alamos National Laboratory that uses Bayesian statistical methods is also described briefly.