Figure 2.

Variation in the percentage clonogenic survival of V79-4 cells with changes in the duration of radiation exposure, for doses in the range 12–20 Gy of cobalt-60 γ-rays. For all doses there is a reduction in efficacy of a given dose as the period of exposure is progressively increased. Thus, for a given effect the dose needs to be progressively escalated in order to maintain a given level of effect. For example, for an exposure of 12 Gy in 30 min the dose would need to be increased to 13 Gy for a 60 min exposure period and to a little over 14 Gy if the exposure period was increased to 120 min. All irradiations were carried out with the cells at 37^oC.