Abstract
Over the next few years the International Commission on Radiological Protection (ICRP) will publish a series of reports containing updated biokinetic and dosimetric models and dose coefficients for occupational intake of radionuclides. The biokinetic modeling scheme continues a trend in modern ICRP reports toward physiologically realistic descriptions of the time-dependent behavior of absorbed radionuclides and ingrowing chain members. This paper proposes systemic biokinetic models for cesium isotopes and their chain members for use in these ICRP reports and examines dosimetric implications of the proposed models. Comparisons of A = tissue dose per unit input to blood based on current ICRP models for workers (ICRP Publication 68, 1994) with B = corresponding values based on the proposed biokinetic models (but using dosimetry models of Publication 68) yields the following ranges of ratios B:A for tissues addressed in current ICRP documents: 0.5-25 for 130Cs (T1/2 = 29.2 min), 0.6-9.5 for 134mCs (2.9 h), 0.8-2.2 for 129Cs (32.1 h), 0.7-1.7 for 131Cs (9.69 d), 0.8-1.3 for 136Cs (13.2 d), 0.7-1.1 for 134Cs (2.06 y), 0.5-1.9 for 137Cs (30.2 y), and 0.2-3.7 for 135Cs (2.3x106 y). The large differences in estimated tissue dose for some tissues and cesium isotopes, particularly short-lived isotopes, result mainly from differences in model predictions of the time-dependent distributions of cesium in the body. For example, the proposed and current ICRP models for cesium predict peak kidney contents of ~22% and ~0.4%, respectively, following intravenous injection of stable cesium. Based on the proposed models for cesium and its progeny, the only dosimetrically significant chain members of cesium isotopes are 137mBa, which represents 32-85% of the estimated tissue doses from injected 137Cs, and 134Cs, which represents 4-53% of the estimated tissue doses from injected 134mCs.
