Non-reparable DNA strand breaks were measured in X-irradiated CHO cells by means of the alkaline unwinding technique and were compared with cell survival measured by the colony assay. The experiments were performed with cells at passage numbers 10, 50 and 110 after thawing from stock culture. Cellular radiosensitivity was found to be identical for all three passage numbers used. By contrast, the dose-response of non-reparable DNA strand breaks was only the same for passage numbers 10 and 50 but significantly steeper for cells irradiated in passage number 110. The ratio of non-reparable breaks to lethal events, as calculated from the survival curves, was found close to 1:1 for cells irradiated at passage numbers 10 and 50 but increased to 20:1 at passage number 110. These data indicate that the number of non-reparable strand breaks measured after irradiation not only depends on cellular radiosensitivity but also on other parameters such as the age of the cell culture.
