Reevaluation of the committed dose equivalent from 232Th and its radioactive progeny.

Multicompartmental models were used in ICRP Publication 30 to describe the metabolism of radioactive elements and their retention in specific organs and tissues. Despite their use of more complicated and sophisticated metabolic models than those used in its earlier ICRP Publication 2 in 1959, the ICRP assumed that the radioactive progeny of 232Th that are produced in the body metabolize like their parents in its Publication 30. This assumption was made for mathematical simplicity and out of necessity when organs and tissues named for the parent are not included in the model of the progeny. This simplifying assumption can lead to overestimates of doses to tissues, especially the critical cells on bone surfaces. More realistic metabolic models and parameter values for 232Th and its radioactive progeny have been developed to estimate the total committed dose equivalent from 232Th and all of its radioactive progeny per unit intake of 232Th only. It is believed that this research has led to (1) more realistic estimates of doses from 232Th and its radioactive progeny over any applicable period of time after an intake, (2) more appropriate derived limits, and (3) metabolic models that can be used in the design of bioassay programs.