RELATIONSHIPS BETWEEN MITOTIC DELAY AND THE DOSE-RATE OF X-RADIATION

Upon exposure of cells to radiation delivered at a continuous low dose rate, cell proliferation may be sustained with the cells exhibiting a constant doubling time that is independent of the total dose. The dou bling time or mitotic delay under these conditions has been shown to d epend on the dose rate in HeLa, V79 and P388F cells (Mitchell et al., Radiat. Res. 79, 520-536, 1979; Fox and Gilbert, Int. J. Radiat. Biol. 11, 339-347, 1966). Re-analysis of the data for these particular cell lines shows that there is a threshold dose rate for mitotic delay, an d that above the threshold there is a linear relationship between the length of mitotic delay and the logarithm of the dose rate which is re ferred to as the dose-rate response. We have observed the same relatio nships for L5178Y (LY)-R and LY-S cells exposed to low-dose-rate radia tion. The threshold dose rates for LY-R, LY-S and P388F cells are simi lar (0.01-0.02 Gy/h) and are much lower than for V79 and HeLa cells. T he slope of the dose-rate response curve is the greatest for HeLa cell s, followed in order by LY-S, V79 and P388F cells, and finally by LY-R cells. The slopes for HeLa and LY-R cells differ by a factor of 35.