Cellular response to low LET radiation

Several models for cell killing by radiation had been offered by many authors: target model, linear quadratic model and so on. One of them is a linear quadratic model (LQ model) which may be applicable to radiation therapy, because it has been demonstrated that not only cellular responses but also normal tissue including tumor tissue to radiation can be expressed by the same factor in LQ model (α/β ratio). Moreover, the studies on LQ model in vitro and in vivo can allow to improve the regimens of radiation therapy like hyperfractionation. Also radiation therapy is, in general, carried out according to the regimens of fractionated irradiation of low LET radiation. Therefore, the experimental data for cell killing by fractionated irradiation would provide basic knowledge in the study of cancer radiation therapy. Thus, cellular responses to fractionated irradiation were examined by using 3 cell lines: Hela cells, V79 cells and L cells. When the cells were exposed to fractionated doses of x-rays with 2 Gy per fraction per day, Hela cells were most radiosensitive and V79 cells were most radioresistant in terms of cell killing. The difference in effect of cell killing was enlarged between two cell lines in the fractionated irradiation of 1 Gy per fraction with a daily dose of 2 Gy at 3 hr intervals. The radiosensitivity of L cells was always median among 3 cell lines. In the analysis of flow cytometer, G2 block for V79 cells and G1 accumulation for Hela cells were major in the cell populations after 3 fractions of the irradiation (2 Gy per fraction). However, L cells showed both G1 and G2 block during the same fractionation. (author)