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Project ID: FIKR-CT-2001-00164
Financiado con arreglo a: FP5-EAECTP C
País: Germany

Implementation of bioassay methods to improve assessment of incorporated radionuclides

The work which was carried out within the work package "bioassay" shall provide commonly acceptable guidelines for optimum performance of inductively coupled plasma mass spectrometry (ICP-MS) measurements with main focus on urinary measurements of uranium, thorium and actinides. From the results of this work following recommendations can be made:

Sampling of urine:
24 hours urine sampling should be collected to avoid large uncertainties in the quantitation of daily urinary excretion values. Wherever possible samples for three consecutive days should be collected and a number of aliquots should be drawn out of total sample for analysis. Care should be taken to avoid external contamination of samples by collecting samples in pre-cleaned sample polyethylene bottles. For storage, urine samples should be acidified and kept frozen before analysis.

Sample preparation:
Pre-dilution is of advantage where sensitivity of the instrument is adequate, in order to avoid obstruction of the cones of the transfer system from the ICP torch to the vacuum of the mass spectrometer. Salt removal from urine samples might be advantageous under certain circumstances but extreme care should be taken during handling and chemical processing to ensure that the reagent blank is extremely small. Once the salt removal chemical procedure with extremely low blank is established, only then it should be adopted. Otherwise the normal dilution procedure should be persisted with.

In addition, several chemical blanks must be prepared simultaneously during the entire sample preparation procedure. At least 3 aliquots of each urine sample should be analysed to take into account any possible variations during the measurement by ICP-MS technique. Reagent blanks should be run with every sample, they should be quantified and correction due to these blanks should be applied. Measurement of total U in urine by ICP-MS at physiological levels (< 10ng/L) requires no sample preparation besides UV photolysis and/or dilution. At these concentrations levels, reliable measurements of 235U (or 235U/238U ratios) require pre-concentration steps such as destruction of organic material of the sample using microwave digestion and selective chemical purification using UTEVA resin. The advantage of using ICP-MS is that the measurement time is considerably shortened (10 minutes versus several days for alpha spectrometry).

ICP-MS technique:
ICP-MS measurements are simple, rapid and economical. New improved measuring techniques (HR-SF-ICP-MS) with detection limits in urine of 150pg/L for 238U, 30pg/L for 235U and 100pg/L for 232Th, respectively, are recommendable and offer much lower detection limits than alpha spectrometry. This method should become the routine technique monitoring of workers and of members of the general public for U isotopes incorporated in the human body.

In addition to the technical questions of bioassay measurements, following substantial points have to be considered. The calculation of internal doses from incorporated radionuclides depends critically on the reliability of the biokinetic models employed. Preferably, these models should be based on sound physiological information and the parameters should be derived from experimental investigations on humans. However, even for radiologically important elements like U, Th, radium or lead, there are inconsistencies in the currently adopted biokinetic models. There is therefore a persisting need for further testing and improvement of these models. This can either be achieved through controlled experimental investigations or by testing the models against monitoring data. Thus, models and measurements may alternatively be used to validate each other. All of these aspects are addressed in this project. The model predictions for U and Th were calculated for various exposure scenarios and were compared with bioassay results, which were done in this project and autopsy data available in the literature. Individual monitoring and biokinetic modelling for a Finnish family due to natural U in drinking water was performed. Measured excretion was compared with model prediction. It was found that reasonable results are achieved using the ICRP biokinetic model for U against measured data.

However, a discrepancy was found between the model and measurement for Th in excretion as well as in bone and whole body. It is proposed that a higher gastrointestinal absorption value might be separately given for ingested Th. Those recommendations might contribute to a more reliable monitoring of incorporated radionuclides in practice and further improve realistic dose coefficients of U and Th in internal dosimetry.

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Uwe OEH, (Deputy Head of Medical Physics Group)
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