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Retrospective assessment of exposures to ionising radiation, ICRU Report 68. By ICRU, pp. 144, 2002 (Nuclear Technology Publishing, Ashford, UK) ISBN 1 870965 94 9

In keeping with previous ICRU reports, Report number 68 is the distilled wisdom of an International Committee of Experts on a specific radiological problem, in this instance the retrospective assessment of human exposure to ionising radiation.

This is a highly specialized subject and under normal circumstances is best left in the hands of the experts who are well able to deal with the small number of cases or situations where retrospective assessment is necessary. However, in the improbable but not impossible event of widespread elevated levels of radiation, the expert services are rapidly overwhelmed, as was well illustrated by the Chernobyl incident. Therefore it is prudent for the wider radiological community to have some knowledge of the subject and this report provides an excellent up-date on the present situation.

10 basically different methods of assessment are discussed, subdivided into radiation measurements performed on individuals and environmental measurements/modelling. In the former category electron paramagnetic resonance (EPR) measurements on the enamel powder from teeth remains one of the most important methods, as also are measurements of chromosomal aberrations in human lymphocytes, either conventional dicentric analysis or more sophisticated techniques for identifying translocations such as fluorescence in situ hybridization. Other methods that can be used on cultured cell samples include assay of micronuclei in human lymphocytes and somatic mutation assays. For estimating the dose from radionuclides within the body, either direct measurement with a whole body counter or radiometric analysis of excreta samples or blood are the methods of choice.

The above approaches are normally the best for determining the dose to individuals but the determination of average doses to larger population groups is usually done by making measurements in the environment and applying appropriate models. Methods falling into this category include measuring the residual luminescence, post irradiation, of a wide range of materials, accelerator mass spectrometry and direct assay of radionuclides in the environment that can give rise to both external and internal exposures.

There is a useful table showing the minimum detectable dose with each method. This ranges from about 1 mGy for whole body counting under favourable conditions (shortly after intake) to about 300 mGy for translocations and micronuclei.

Most of the methods perform reasonably well for acute whole body doses of gamma radiation shortly after irradiation, but in other scenarios there are many potentially confounding factors. These include type of radiation, time course of exposure, time of measurement after exposure, partial body exposure, and in some cases, variation in base-line (zero dose) values with age. Perhaps for these reasons there is very little information on the accuracy of the various methods, ±25% at 400 mGy is quoted for EPR of tooth enamel.

Recommended methods in different circumstances are summarized below:

• Individual exposures after intake of radionuclides with measurable content of radionuclides in the body at time of dose reconstruction — measurements of radionuclides in the body.
  
• Individual exposures with largely uniform dose distribution in the body within one year of exposure — dicentric chromosomes.
  
• Individual exposures with largely uniform dose distribution in the body at a time period of 1–10 years after exposure — EPR of teeth or chromosomal translocations.
  
• Individual exposures with largely uniform dose distribution in the body more than 10 years after exposure — EPR with teeth.
  
• Doses to population groups due to external exposures in a small area — luminescence methods.
  
• Doses to population groups in a large area — modelling based on environmental measurements.
  

There is a comprehensive list of references, a useful glossary of terminology and an index.

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