A critical review of 55Fe dosimetric models

The available literature on 55Fe dosimetry has been devoted to environmental exposures and medical iron kinetic studies. For occupational dosimetry, ICRP (1979, 1988) published a non-recycling dosimetric model for iron. These ICRP publications do not provide information on iron excretion. Johnson and Dunford (1985) published dose conversion factors and urinary excretion curves based on the ICRP (1980) and MIRD (1983) iron metabolic models. A critical review of these models was undertaken to select a model for occupational dose assignment. The review indicated that the information and recommendations in ICRP (1988) and Johnson and Dunford (1985) are dependent on unrealistic assumptions that do not agree with known iron metabolism. Therefore, an alternative model is proposed for dosimetric application. Calculations of dose conversion factors and urinary excretion curve for class W55Fe inhalation exposure (1 micron AMAD) using the proposed model are compared with predictions based on ICRP (1980) and Johnson and Dunford (1985) models. The difference in the practical outcome (i.e., dose assignment) is examined by applying the proposed and reviewed models to a realistic bioassay case. The Johnson and Dunford (1985) model yields a dose estimate which is roughly a factor of ten higher than values predicted by ICRP (1980) and the proposed model. Some of the disagreement is due to uncertainty in the fraction of radio-iron excretion via urine. Further research on this subject is recommended. In the interim, the proposed model is recommended for occupational dose assignment.