The effect of X-ray dose rates on low-dose HRS phenomenon with cells in vitro

The researches about risk of low dose radiation is still going on. HRS phenomenon is studied actively as part of low dose radiation research. HRS is that mammalian cells exhibit higher sensitivity to radation at low doses than expected by the linear-quadratic model when clonogenic assay is done. The mechanisms about HRS is verified well, but studies about the relationship between HRS and radiation dose rates have no consistency. Therefore, in this study, the effect of radiation dose rates on HRS is verified with in vitro experiments. Clonogenic assay is conducted with rat diencephalon and rat gliosarcoma with 0.5 Gy/min dose rate condition and HRS was observed with this condition in both cell lines. The dose rate was changed to 0.125 and 1.5 Gy/min. As a result, HRS was observed with 0.125 Gy/min radiation but not with 1.5 Gy/min in both cell lines. It is expected that the difference of repair mechanisms with different dose rates is the key point of above experiments results. To verify the assumption, the two-step irradiation clonogenic assay is done. After priming irradiation with 0.2 Gy, 1.5 Gy/min condition, the HRS phenomenon was eliminated when irradiated with 0.125 and 0.5 Gy/min dose rates in both cell lines. Through this result, it is analyzed that high dose rate irradiation can induce repair mechanisms even at low dose range. Lastly, induced apoptosis with different dose rates was observed with flow-cytometry experiment. With low dose rate conditions that HRS is seen, apoptosis was induced more than high dose rate condition in both cell lines. It is interpreted that when cells are irradiated with high dose rate low dose radiation the repair mechanisms were induced so apoptosis did not occur. In this study, the different conclusion is made compared with preceding researches in relationship between HRS and radiation dose rates. And it is confirmed that apoptosis cell death is a important factor in HRS response, therefore there is little reason to overestimate the biological effect of low dose radiation exposure with HRS phenomenon. Finally, in the case of fractionated radiation therapy for cancer treatment, proper dose rate that can prevent HRS response could be selected to protect normal cells from unnecessary cell death