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

Abstract

Unfed plateau-phase cells were irradiated with either single doses or up to 10 fractions of X-rays 6 h apart. The single-dose survival curve had an extrapolation number of 11.4, and the O2-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 5 fractions of X-rays, but tended to bend downwards with increasing number of fractions. Cells that had survived 5 fractions of 200 rad (or 600 rad in hypoxia) 6 h apart, were less able to absorb damage as sub-lethal than those not previously 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.