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A cobalt-60 irradiation accident occurred in Shanxi,China On April 11,2008.Five victims were exposed to whole body dose ranging from 1.7-14.5Gy,respectively.Two victims were dead at 62 days and 1.5 years post-irradiation.The other three victims received medical follow-up and were observed continuously until fifth year with multiple cytogenetic analyses.The unstable chromosome aberrations including dicentric and centric ring(dic+r)and the micronuclei frequency in binucleated lymphocytes were detected.In addition,G-banding karyotype and fluorescence in situ hybridization(FISH)methods were used to analyze translocations for exploring the chromosome stability and retrospective dosimetry.The results showed that the unstable chromosome aberration,dic+r declined each year,dropping to about 20-40%of initial levels by the 5th year.A similar trend was observed for the micronuclei frequency.G-banding karyotype analyses showed that the translocation rates for the victims were remained relatively stable during 5 years and were similar with FISH analyses results at 5th year after irradiation.The retrospective estimated doses which were reconstructed based on the translocation rates were consistent with the biological doses estimated with dic+r at first day post-irradiation.The results of this study indicate that the stable chromosome aberration is an ideal index for early irradiation dose reconstruction and evaluation of radiation effects.