Recovery from x-ray induced damage in human cells grown in culture.

Cells from an established human cell-line, NHIK 3025, originally derived from an early stage of cancer of the cervix, were tested for recovery capacity when irradiated with X-rays under aerobic (equilibrated with air) and extremely hypoxic conditions (O2 content less than 4 ppm). The dose-response curve obtained under aerobic conditions had a D0-value of 130 rads and an extrapolation number of 3.8. The corresponding curve obtained with a first dose exceeding that of the shoulder region of the survival curve followed 3 hours later by graded second doses, had a D0-value of 130 rads and extrapolation number 2.2. The curve obtained by measuring the survival after two equal doses of 330 rads, each with variable time intervals between the doses, showed a time dependent radiosensitivity. The dose-response curve obtained by irradiating the cells under extremely hypoxic conditions was well fitted by an exponential line up to about 2500 rads followed by a downward bend. Recovery from sublethal damage was not observed in split-dose experiments where the total doses were less than 2500 rads. For a total dose of 4200 rads, however, a split-dose effect was observed (SDR = 1.6). This split-dose effect is probably not due to Elkind repair, but is rather a consequence of the technique used.