Biological dosimetry of atomic bomb survivors exposed within 500 meters from the hypocenter and the health consequences.

Seventy-eight atomic bomb survivors were examined for biological dosimetry using chromosome abnormality. They had been exposed within 500 meters from the hypocenter in heavily shielded conditions and were found from NHK-RIRBM joint study carried out from 1966 to 1971. Estimation of the exposure doses for these survivors was made under the following steps; 1) calculation by DS86 system (physically estimated doses) in survivors who had been exposed within 1,500 meters and had precise records of exposure conditions. RBE for the neutron was defined as 10. 2) setting of exposure dose-chromosome aberration curve, and 3) observation of chromosome aberrations in the proximally exposed survivors, for whom biological doses were estimated. Estimation of the exposure doses were possible from the aberration rate of chromosome in the peripheral lymphocytes, even 25-40 years after the exposure. Of the 78 survivors, 96% were estimated to have exposed more than one Sv. Detection of transforming gene(s) of N and K RAS genes in DNAs from non-leukemic survivors was carried out as one of the biological investigations for these heavily exposed survivors. All four survivors examined showed N or K RAS gene mutation. Three of the four healthy survivors had cancer or leukemia 7-10 years after the examination. Further continuous follow-up study of these heavily exposed people will give us more information on the late effects of A-bomb radiation, which may arise in the future.