Comparison of the ICRP and MIRD models for Fe metabolism in man

A task group of the Medical Internal Radiation Dose (MIRD) Committee has recently published a model of Fe metabolism in man. This model was developed to calculate doses from radioiron injected for medical diagnostic purposes. It is a compartment model with recirculating Fe exchanging between plasma and extracellular fluids, tissue storage compartments, bone marrow and red blood cells (RBC). It is a first order model with the exception of Fe in the RBC compartment, which is assumed to retain Fe for 120 days, at which time the Fe returns to the extracellular fluid compartment. By contrast, the International Commission on Radiological Protection (ICRP) model is a "once through" first order compartment model, with the compartments represented by organs (spleen, liver and other soft tissue) rather than physiological compartments as in the MIRD model. Both of these models have been implemented in the computer code GENMOD which contains the ICRP recommended lung and gastrointestinal tract models and which is used at the Chalk River Nuclear Laboratories to calculate doses, excretion rates, derived investigation levels, etc. The results of calculations using these models have been compared to see if the much less sophisticated ICRP model was adequate for radiation protection purposes. It was found that the effective dose per unit intake of radioiron was higher for the MIRD model and urinary excretion rates following an exposure were considerably different. It is concluded that the ICRP model should not be used in dosimetry calculations, or for comparing monitoring results to model calculations.