A recycling systemic model for the biokinetics of molybdenum radionuclides

An overview of the available data on molybdenum biokinetics and metabolism in humans is presented, with special emphasis on the results of stable tracer studies conducted in recent years, after the publication of the systemic model for incorporated radionuclides of molybdenum recommended by the International Commission on Radiological Protection (ICRP). On the basis of the presented information, a new structure for a compartmental model of molybdenum biokinetics, including the return of material from the organs back to the systemic circulation, was developed. The structure chosen is a compromise between the attempt to provide a realistic description of the biokinetics and the need to have a simple tool for dose estimation. The model consists of two compartments associated to the extracellulare fluids (blood plasma and interstitial fluids), liver, kidneys, and one generic compartment to represent all other tissues. The possibility of a direct excretion pathway into the urine was introduced, in order to correctly describe the rapid excretion as observed in the human studies. Reference values of the model parameters have been estimated taking into consideration that the amounts of radioactive molybdenum accidentally incorporated are negligible in comparison to the daily dietary intake of stable molybdenum.