Establishment of a dose-response curve of 60Co gamma-ray irradiation by dicentric chromosome analysis

Ionizing radiation exposure causes DNA strand breaks that lead to chromosome aberrations. Among radiation-induced chromosome aberrations, multicentric chromosomes, as represented by dicentric chromosomes, are considered to be sensitive and specific biomarkers for assessing the radiation dose. For more than three decades, dicentric chromosome analysis (DCA) has been the Gold Standard of biodosimetry. In DCA, a dicentric yield per cell of a radiation-exposed patient is applied to a calibration curve (dose-response curve). Many dose-response curves have been proposed thus far, but most of them have been generated respectively from one healthy donor by one in vitro experiment. According to the result of international collaborative works on inter-laboratory comparisons, it has been reported that in DCA, the significant variation of dose estimation is partly attributed to different experimental protocols including the scoring criteria of chromosome aberrations among institutions / investigators. Thus, it is necessary for a biodosimetry laboratory to have and use its own dose-response curve under its own experimental conditions. In the present study, in order to evaluate the dose estimation of patients for better medical preparedness, a dose-response curve was established by analyzing 13 occupationally non-exposed healthy volunteers by DCA using peripheral blood samples irradiated in vitro with 60Co gamma-rays at seven different doses (0, 0.5, 1.0, 2.0, 3.0, 4.0 and 5.0 Gy). The result of the first-division metaphase scoring followed a linear-quadratic equation, Y=A+aD+bD2 (Y: the yield of dicentrics, D: the dose, A: the background frequency, a: the linear coefficient, b: the dose squared coefficient; IAEA Technical Reports Series No. 405 Cytogenetic Analysis for Radiation Dose Assessment, Vienna, 2001). Then, we established a practical biodosimetry protocol for radiation emergency medicine, tentatively called the NIRS DCA System (including sample collection, cell culture, chromosome preparation, automated metaphase image-capturing, chromosome aberration scoring in triage- and full estimation-modes, and a diagnostic report format), for conducting dose estimation within several days of receiving blood samples. The NIRS DCA System is now being used for actual radiation exposure accidents in Japan.the 13th International Congress of the International Radiation Protection Associatio