The effect of repeated small doses of radiation on recovery from sub-lethal damage by Chinese hamster cells irradiated in the plateau phase of growth.

Unfed plateau-phase cells have been irradiated with either single doses or up to ten fractions of X-rays 6 hours apart. The single-dose survival curve had an extrapolation number of 11-4, and the oxygen-enhancement ratio (o.e.r.) was 3-1. Cells were exposed to multiple fractions of 200 rad or 150 rad in air and 600 rad or 450 rad in hypoxia. The resulting survival curves did not fit a multi-target, single-hit model of cell survival, being much steeper than that would predict. The curves were exponential up to five fractions of X-rays, but tended to bend downwards with increasing number of fractions. Cells that had survived five fractions of 200 rad (or 600 rad in hypoxia) 6 hours apart, were less able to absorb damage as sub-lethal than those which had not previously been exposed to radiation. The ratio of the initial slopes of the fractionated survival curves for irradiation in air and hypoxia was 2-1, implying that the o.e.r. "on the shoulder" may be less than that in the exponential region of survival.