Modelling the behaviour of chains of radionuclides inside the body

For many radionuclides, estimates of dose to radiosensitive tissues depend strongly on assumptions regarding retention of decay products, but the sparsity of experimental information on individual decay products has led to the use of blanked assumptions in metabolic models. In ICRP Publication 2 each radioelement born in an organ is assigned the retention function derived from that element assuming its direct deposition into that organ from blood. In ICRP Publication 30 daughter products generally are assumed to remain with the parent. Limited experimental data indicate that both approaches may underestimate dose for some important radionuclides. In this paper we examine problems related to modelling the retention of decay products. We discuss how the characterisation of the retention, translocation, and excretion of the daughter as well as that of the parent may be improved in many cases by using mechanistic models that accomodate not only element-specific information but also basic physiological information concerning the skeleton or other tissues.