Host: The Japan Radiation Research Society, Chairman of the 52nd Annual Meeting, Toshiteru Okubo (Radiation Effects Research Foundation)
Exposure to radiation rarely causes immediate signs or symptoms after exposure. However, prodromal symptoms could appear soon when a whole body is exposed to high dose radiation for a short duration. A criticality accident, which occurred at Takaimura of Ibaraki prefecture, Japan in 1999, caused exposure of three people to γ-rays and neutrons. They developed prodromal symptoms soon after exposure. In this accident, thus, three victims were heavily exposed. Dose assessment is performed by the biological/medical method and the countermeasures based on radiation physics and calculation. However, most of the data, such as the dose rate of radiation, its distribution, and the quality needed to evaluate the average dose, are not available when the decision for treatment should be made. In the accident, therefore, the temporary dose estimation of these victims was performed from the onset time and severity of prodromal symptoms. More detailed doses were assessed later by serial changes of lymphocyte numbers, chromosomal analysis, 24Na, 45Ca, or 32P activity, and theoretical calculation. The results obtained from these methods were fairly consistent. One of the problems in dose assessment for the exposure to γ-rays and neutrons is how to evaluate the effect of neutrons on victims and the contribution of neutrons in the exposure. Considering the relative biological effectiveness (RBE) of the neutrons to be 1.7 from our own results and the ratios of neutrons to γ-rays 1.4-1.9, we estimated doses of 3 victims as 16-25, 6-9, and 2-3 grays equivalent as biological doses (GyEq), respectively. The other problem is that the accidental exposure is always inhomogeneous. Therefore, the estimated average dose of whole body is not necessarily in parallel to symptoms or signs previously reported. From the accident we have learned that there is no identical accident to other and that dose assessment from various methods is required for evaluation of damages to the body is required.
