Improvement of biological dosimetry by cytogenetics for operational purposes. Dicentrics or micronuclei?

Voisin, P.; Paillole, N.; Benderitter, M.; Claraz, M.; Chambrette, V.; Delbos, M.; Durand, V.; Leroy, A.; Sorokine-Durm, I. 10. Proceedings of the 10th international congress of the International Radiation Protection Association on harmonization of radiation, human life and the ecosystem, Japan Health Physics Society May 2000 1 v. [6 p.] , No.P-3a-217

The biological dosimetry is an important part of the diagnostic and the prognostic in case of a overexposure suspicion to ionizing radiations. It intervenes besides the clinical symptoms and the physical reconstruction, in order to assist the medical team in charge of irradiated patients for adopting the best therapy strategy. The reference technique in biological dosimetry remains the scoring of unstable chromosome aberrations (dicentrics, centric rings and excess acentrics) in blood peripheral lymphocytes. However, other cytogenetic endpoints such as micronuclei scoring in binucleated lymphocytes could constitute a credible alternative, because most quicker and easier than dicentrics assay. Whatever the technique used, the dose assesment cannot be approached in the same way for expertise of overexposure suspicion or for triage of a large accident. For the former, quality and precision of assessment are needed and for the latter, speed and efficiency are the most important. To combine these different aims, improvement of the dosimetry by cytogenetics was developed in our laboratory using two different methods, dicentrics and micronuclei assays. Special procedures for the simultaneous preparation of several tens of blood samples and for the aberrations scoring with a reduced number of cells were designed and tested in several simulated accidents. For dicentrics assay, it was shown that 100 blood samples can be routinely prepared and scored in 7 days, and the observation of 50 metaphases per sample allows a checked precision of the dose estimate of +-1 Gy (95% confidence interval). Moreover, to improve dicentrics scoring, an image analysis system in under development in our laboratory. The first results show a two-fold improvement of scoring speed in the semi-automatic version and a good comparison with other existing systems. In similar experimental conditions, micronuclei test could be performed in 7.5 days, and micronuclei could be scored in 500 binucleated cells with the same delay than 50 metaphases. The direct consequence of above is that the estimate uncertainty of the dose is improved by a factor 2 (+-0.5 Gy of confidence interval). The lack of specificity of micronuclei for low doses of radiation is not a limiting factor in case of large accident, but for expertise only. Finally, micronuclei test could usefully replace dicentrics assay for screening large population, especially if non cytogenetician specialists are involved.