Assessing the reliability of dose coefficients for exposure to radioiodine by members of the public, accounting for dosimetric and risk model uncertainties

Assessments of risk to a specific population group resulting from internal
exposure to a particular radionuclide can be used to assess the reliability of the
appropriate International Commission on Radiological Protection (ICRP) dose
coefficients used as a radiation protection device for the specified exposure
pathway. An estimate of the uncertainty on the associated risk is important for
informing judgments on reliability; a derived uncertainty factor, UF, is an
estimate of the 95% probable geometric difference between the best risk
estimate and the nominal risk and is a useful tool for making this assessment.
This paper describes the application of parameter uncertainty analysis to
quantify uncertainties resulting from internal exposures to radioiodine by
members of the public, specifically 1, 10 and 20-year old females from the
population of England and Wales. Best estimates of thyroid cancer incidence
risk (lifetime attributable risk) are calculated for ingestion or inhalation of 129I
and 131I, accounting for uncertainties in biokinetic model and cancer risk
model parameter values. These estimates are compared with the equivalent
ICRP derived nominal age-, sex- and population-averaged estimates of excess thyroid cancer incidence to obtain UFs. Derived UF values for ingestion or
inhalation of 131I for 1 year, 10-year and 20-year olds are around 28, 12 and 6,
respectively, when compared with ICRP Publication 103 nominal values, and
9, 7 and 14, respectively, when compared with ICRP Publication 60 values.
Broadly similar results were obtained for 129I. The uncertainties on risk estimates
are largely determined by uncertainties on risk model parameters rather
than uncertainties on biokinetic model parameters. An examination of the
sensitivity of the results to the risk models and populations used in the calculations
show variations in the central estimates of risk of a factor of around
2–3. It is assumed that the direct proportionality of excess thyroid cancer risk
and dose observed at low to moderate acute doses and incorporated in the risk
models also applies to very small doses received at very low dose rates; the
uncertainty in this assumption is considerable, but largely unquantifiable. The
UF values illustrate the need for an informed approach to the use of ICRP dose
and risk coefficients.