Biokinetics of ingested radionuclides and assessment of internal dose

The release of natural and artificial radionuclides into the environment is increasing the need of being able to assess radiation dose to the members of the public. To obtain reliable estimates of the doses to a population, information is required on the uptake of radionuclides following ingestion and/or inhalation, on their internal distribution, excretion and on factors influencing their biokinetics. A new physiologically based model of the gastro-intestinal tract is needed, by taking into consideration recent data on gut transit times (Stubbs, 1992) and crossing them with factors such as age and gender. The selection of biokinetic parameters to be used in such a model calls primarily for data obtained on humans. Methods, using stable isotope and, when necessary, chemical analogues for radiologically important elements which have no stable isotopes, offer great opportunities, especially for studies on gastro-intestinal absorption. The first applications of stable isotopes for studies of metabolic processes, essentially related to nutrition and food science, are dating back to the forties and considerably expanded for the basic elements H, O, C and N till the beginning of eighties when several groups concerted efforts in various aspects of metabolism of essential minerals. Two general methods are available for analysis of stable isotopes and both of them are based on the differences in their nuclear properties: mass spectrometry analysis and activation analysis. Inductively coupled plasma mass spectrometry (ICP-MS), thermal ionization mass spectrometry (TIMS) and activation analysis as NAA and CPAA may be referred as complementary techniques for stable isotope determination in biological samples. Human metabolic data, obtained up to now, by using stable isotopes of elements of radiological significance such as Mo, Sr, Ba, Nd, showed that fractional gut absorption of some elements may exceed the adopted ICRP values. Further investigations on these elements, on Te, Zr, Ru and on selected elements of the lanthanide series, taken as actinide analogues, are currently carried out. The methods for quantifying isotopes are time consuming and demand equipment and skills which are restricted to few laboratories. Nevertheless the use of stable isotopes is the only answer fitting the need of an ethically justifiable and publicly acceptable, method for investigating a population.