Biokinetic and dosimetric modelling in the estimation of radiation risks from internalemitters

Abstract

The International Commission on Radiological Protection (ICRP) has developed biokineticand dosimetric models that enable the calculation of organ and tissue doses for a wide range ofradionuclides. These are used to calculate equivalent and effective dose coefficients (dose inSv Bq−1 intake), considering occupational and environmental exposures. Dose coefficients have alsobeen given for a range of radiopharmaceuticals used in diagnostic medicine. Usingequivalent and effective dose, exposures from external sources and from differentradionuclides can be summed for comparison with dose limits, constraints and referencelevels that relate to risks from whole-body radiation exposure. Risk estimates are derivedlargely from follow-up studies of the survivors of the atomic bombings at Hiroshima andNagasaki in 1945.New dose coefficients will be required following the publication in 2007 of new ICRPrecommendations. ICRP biokinetic and dosimetric models are subject to continuing reviewand improvement, although it is arguable that the degree of sophistication of some of themost recent models is greater than required for the calculation of effective dose to areference person for the purposes of regulatory control. However, the models arealso used in the calculation of best estimates of doses and risks to individuals, inepidemiological studies and to determine probability of cancer causation. Models arethen adjusted to best fit the characteristics of the individuals and populationunder consideration. For example, doses resulting from massive discharges ofstrontium-90 and other radionuclides to the Techa River from the Russian Mayakplutonium plant in the early years of its operation are being estimated usingmodels adapted to take account of measurements on local residents and otherpopulation-specific data. Best estimates of doses to haemopoietic bone marrow, in uteroand postnatally, are being used in epidemiological studies of radiation-induced leukaemia.Radon-222 is the one internal emitter for which control of exposure is based on directinformation on cancer risks, with extensive information available on lung cancer inductionby radon progeny in mines and consistent data on risks in homes. The dose per unit 222Rn exposure can be calculated by comparing lung cancer risk estimates derived for 222Rn exposure and for external exposure of the Japanese survivors. Remarkably similar valuesare obtained by this method and by calculations using the ICRP model of the respiratorytract, providing good support for model assumptions. Other informative comparisons withrisks from external exposure can be made for Thorotrast-induced liver cancer andleukaemia, and radium-induced bone cancer. The bone-seeking alpha emitters,plutonium-239 and radium isotopes, are poorer leukaemogens than predicted by models.ICRP dose coefficients are published as single values without consideration of uncertainties.However, it is clear that full consideration of uncertainties is appropriate when consideringbest estimates of doses and risks to individuals or specific population groups. Anunderstanding of the component uncertainties in the calculation of dose coefficients can beseen as an important goal and should help inform judgements on the control of exposures.The routine consideration of uncertainties in dose assessments, if achievable, would be ofquestionable value when doses are generally maintained at small fractions of limits.