Basis and implications of the ICRP's new biokinetic model for thorium

The International Commission on Radiological Protection (ICRP) has adopted a new, age-specific biokinetic model for thorium, along with new assumptions concerning the fate of radioactive progeny produced in the body after intake of thorium. This paper explains the basis for the model and assumptions and examines some of their implications with regard to radiation dosimetry and bioassay. The new model differs substantially from the ICRP's previous biokinetic model for thorium (introduced in 1979 in ICRP Publication 30) with regard to model structure, sources of data used to derive parameter values, predictions of integrated activities of long-lived thorium isotopes in some radiosensitive organs, and predictions of the rates of urinary and fecal excretion of thorium at times remote from exposure. For the case of injection of ²³²Th into blood of an adult, the new model and assumptions yield 4- to 8-fold decreases in predictions of 50-y committed equivalent doses to radiosensitive skeletal tissues but 7- to 55-fold increases in predictions of committed equivalent doses to extra-skeletal tissues, compared with the model and treatment of decay chain members recommended in ICRP Publication 30.