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On the fake accuracy in radiation protection

Dear Editor, The International Commission on Radiological Protection (ICRP) recently released the Publication 144 entitled ‘Dose Coefficients for External Exposures to Environmental Sources’. It provides conversion coefficients for exposures from environmental radiation sources to effective doses of various age individuals. The coefficients are based on the mean absorbed doses to tissues and organs simulated with age and gender dependent reference person phantoms. It claims that the effective dose of a juvenile externally exposed to radiation from contaminated ground by a nuclear accident could be tens of percent greater than that of an adult. The ICRP’s system of radiological protection has been built on the nominal risk of radiation exposure by averaging risks over a population with typical age distribution, and the effective dose has been calculated with the adult reference person since 2007. Therefore, although it is stated for specific external exposure situations, ICRP seems to extend the concept of the effective dose to a more sophisticated age-dependent one. However, the way of this improvement is quite incoherent since it only reforms the mean absorbed dose of tissues and organs more realistic, i.e. age-dependent while leaving another component of the effective dose, the tissue weighting factor, traditionally age distribution averaged. As is well known, cancer incidence risk of tissues and organs [1, 2] varies with age but the age dependencies are not the same between tissues and organs. So, the age-dependencies of excess cancer incidence risk by radiation exposure differ among tissues and organs. For example, excess lung cancer incidence risk is greatest for themiddle-aged, while that of thyroid cancer decreases relatively quickly with age. Therefore, nominal radiation risk of each tissue and organ varies with age making both their relative values and their total value change with age. This means, different tissue weighting factors should be used if age-dependent change is considered at all, and the sum of age-dependent tissue weighting factors is not always unity. The conversion coefficients of the publication 144 neglected taking into account this point. Consequently, the age-dependent effective dose in Publication 144, at first glance, may seem to make the concept of the effective dose more sophisticated and realistic, but in reality, the improvement is no more than a fake accuracy. The age-dependent effective dose conversion coefficients for internal exposure also has the problem of using fixed tissue weighting factors. However, this age dependence is worth considering for radiation protection since this age difference, being affected not only by body size but also by biokinetics, does not remain below tens of percent but can reach to ten times. The ICRP commented in paragraph 4 of the publication that differences in effective dose-rate coefficients as a function of age only reflect differences in physical size and organ masses, and do not address age differences in detriment. ICRP stated that the primary use of the effective dose is for regulatory applications [2]. The effective dose does not need to be numerically detailed for the purpose, and refining the concept for making a difference of at most tens of percent is meaningless. It is understood that ICRP for this reason defined the quantity in Publication 103 based exclusively on the adult phantom. Despite of the ICRP’s comment, misusing estimated age-dependent effective doses for comparing radiation exposure risk between different age groups is quite liable to occur.