Optimal bioassay time allocations for multiple accidental chronic intakes of radioactive particles
Bioassays are applied to workers and other people exposed to radiation through routine or accidental intake of radioactive isotopes. The quantity of the isotope intake by an individual can be estimated from the measured quantities in the bioassays using the corresponding retention or excretion function. A retention (excretion) function represents the predicted activity of a radionuclide in the body, organ, or tissue or in a 24-h excreta at a particular time after the intake. The International Commission on Radiological Protection provides these biokinetic models. We have used these functions to compute optimal experimental designs for estimating the intake quantity in workers following radionuclide inhalation or ingestion. In particular, we have considered the case where the individual is exposed to a constant intake during some periods followed by other periods without intakes, which we called multiple constant intakes. The main aim of this work is finding the best times where the bioassay samples should be obtained in order to estimate optimally the actual intake of the last period. The problem is also extended to compute the next two or more bioassays to be performed. Measurements on the same worker are modeled through a correlation structure. The outline of the problem is established for a general case and results are given for a typical case as a real example. The methodology given in the paper can be applied to other cases with multiple constant intakes, e.g. in medical treatments where the patient is exposed to successive doses during particular periods.