Dose-rate effects in mammalian cells in culture. III. Comparison of cell killing and cell proliferation during continuous irradiation for six different cell lines

The effects of continuous irradiation over a wide range of dose rates were studied for six different mammalian cell lines in regard to cell survival and proliferation. Cell lines were chosen in which such characteristics as population doubling time, chromosome number, DNA content, acute dose-survival curve parameters, and division delay were as diverse as possible. There was no correlation between the minimum dose rate necessary to stop cell population growth and any of the above listed characteristics, with the exception of division delay following acute doses. In general, the longer the division delay (min/rad), the lower the dose rate required to stop cell population growth. The effects of cell-cycle redistribution during continuous irradiaton in regard to cell survival were dramatic. In some cases a reduction in dose rate resulted in an increase in cell killing for a given total dose. This occurred only when dose rates were sufficient to stop cell population growth and after exposure times sufficient to allow for the occurrence of cell-cycle redistribution