Biokinetic models for radiocaesium and its progeny

The International Commission on Radiological Protection (ICRP) is preparing a series of reports that will provide updated biokinetic and dosimetric models and dose coefficients for occupational intake of radionuclides. The biokinetic modelling scheme continues a trend in ICRP reports towards physiologically realistic descriptions of the time-dependent behaviour of absorbed radionuclides and their radioactive progeny. This paper proposes systemic biokinetic models for caesium isotopes and their ingrowing chain members and examines the dosimetric implications of the proposed models. Comparisons of D₆₈ = tissue dose per unit input to blood based on current ICRP models for workers (ICRP Publication 68, 1994) with D_P = corresponding values based on the proposed biokinetic models (but using the dosimetry models of Publication 68) yields the following ranges of the ratios D_P:D₆₈ for the tissues addressed in Publication 68: 0.5-25 for ¹³⁰Cs (T _1/2= 29.2 min), 0.6-9.5 for ^134mCs (2.9 h), 0.7-1.7 for ¹³¹Cs (9.69 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 (2.3 × 10 ⁶ y). The large differences in the derived dose coefficients for some tissues and caesium isotopes, particularly short-lived isotopes, result mainly from differences in predictions of the time-dependent distributions of caesium in the body. For example, the proposed model and the current ICRP model for occupational intake of caesium predict peak kidney contents of ∼22% and ∼0.4%, respectively, following intravenous injection of stable caesium. Based on the proposed models for caesium and its progeny, the only dosimetrically significant chain members of caesium isotopes with half-life ≥10 min 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.