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

Summary

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.