Influence of dose rate on leukemogenesis in mice irradiated by gamma rays at a high dose rate or low dose rates

The purpose of this study was to assess the extent of the effect of dose rate on tumorigenesis, focusing on myeloid leukemia, one of the typical neoplasias induced by radiation. In the experiment, C3H/He male mice were irradiated with gamma-rays at various doses at a dose rate of 0.88Gy/min, or 0.30mGy/min or 0.016mGy/min, and equations of dose response curves for leukemia were derived from data on the incidence in each group by the least square method. Each equation fitted well with the linear quadratic equation by coefficiency of more than 90%. The extent of the dose rate effect was expressed as dose and dose rate effectiveness factor (DDREF) based on the definition by ICRP-Publication 60. We obtained DDREF by dividing the slope factor of the tangential line of group H (0.88Gy/min) by that of group L-1 (0.30mGy/min) or L-2 (0.016mGy/min), the tangential lines being drawn from the value of the incidence of the control group on the dose response curve. The value of group H to group L-1 was 12.00/1.71=7.022, and that of group H to group L-2 was 12.00/2.56=4.69. These values indicated that reduction of the dose rate caused a decrease in the induction of leukemia, ranging between the values 2 and 10 in the NCRP 1990 report. (author)