Usefulness of continuous probability distributions of rates for modelling radionuclide biokinetics in humans and animals

Abstract

Modelling the biokinetics of radionuclide excretion or retention is important in nuclear medicine and following accidental/malicious radioactivity releases. Sums of discrete exponential decay rates are often used, but we hypothesized that continuous probability distributions (CPD) of decay rates can describe the data more parsimoniously and robustly. We tested this hypothesis on diverse human and animal data sets involving various radionuclides (including plutonium, strontium, caesium) measured in the laboratory and in regions contaminated by the Fukushima and Chernobyl nuclear accidents. We used four models on each data set: mono-exponential (ME) with one discrete decay rate, bi-exponential (BE) with two rates, gamma-exponential (GE) with a Gamma distribution of stretched-exponential rates, and power-decay (PD) with a Gamma distribution of power-decay rates. Information-theoretic model selection suggested that radionuclide biokinetics, e.g. for plutonium in humans, are often better described by CPD models like GE and PD, than by discrete rates (ME and BE). Extrapolation of models fitted to data at short times to longer times was frequently more robust for CPD formalisms. We suggest that using a set of several CPD and discrete-rate models, and comparing them by information-theoretic methods, is a promising strategy to enhance the analysis of radionuclide excretion and retention kinetics.

Enhancing low-dose risk assessment using mechanistic mathematical models of radiation effects

“…A simple alternative involves using CPDs of decay rates [45,46]. This concept in the field of radionuclide biokinetics goes back at least 40 years [47], but has not been used extensively.…”

“…We applied the CPD approach to a variety of human and animal data with different radionuclides (including plutonium, strontium, cesium) measured in the laboratory and in regions contaminated by the Fukushima and Chernobyl nuclear accidents [45,46]. Performances of CPD models with those of sums of discrete rates were compared.…”

“…Consequently, CPD can provide potentially useful parsimonious descriptions of radionuclide biokinetics in a variety of settings relevant for low-dose/dose-rate risk estimation [45,46]. For example, using a set of several CPD and discrete-rate models, and comparing them by information-theoretic methods, is a promising strategy to enhance the analysis of radionuclide excretion and retention kinetics.…”