Host: The Japan Radiation Research Society
Co-host: City of Kitakyushu, University of Occupational and Environmental Health, Japan
Simultaneous inclusion of dose and dose-rate is required to evaluate the risk of long term irradiation at low dose-rates, since biological responses to radiation are complex processes that depend both on irradiation time and total dose. Consequently, to estimate quantitative dose-response relationship on the biological response to radiation, it is necessary to consider a model including cumulative dose, dose-rate and irradiation time. In this study, we measured micronucleus formation and [3H] thimidine uptake in human cells as indices of biological response to gamma radiation, and analyzed mathematically and statistically the data for quantitative evaluation of radiation risk at low-dose and low dose-rate. Our results suggest that the biological response declined sharply when dose-rate was less than 0.01 Gy/h. Furthermore, for defining statistically no risk level, we adapted some statistical models to data at low dose, and compared them with other statistical indexes corresponding to "threshold limit". The values of statistically no risk levels and their confidence intervals depend on definitions of the value and on statistical models. These results suggest that we should statistically define the term "threshold" and include dose rate effects in the risk assessment model. Moreover, we acquired iterative experimental data of biological reaction to low dose gamma radiation and analyzed them using meta-analysis to evaluate quantitatively the radiation effects at very low dose. Synthesized effect sizes of the iterative experiments showed no significant difference between irradiated mice and controls. Our statistical models support biological observations with sufficient statistical power, and numerical expression of our data showed clear dose-rate effect on biological response.
