Figure 4.

Variation in the percentage clonogenic cell survival of V79-4 cells with two doses of 7 Gy of cesium-137 γ-rays, each dose being delivered in approximately 3 min with variable time gaps of 15–120 min between the two doses. The effects of irradiation at 37^oC (■) with those where, over the period of irradiation, cells were either held at room temperature (□) or on ice (▲). The percentage clonogenic survival of cells at room temperature and on ice does not change significantly with changes in the time gap between the two doses of 7 Gy. For irradiation at 37^oC, the loss of efficacy was approximately 2% with a 60 min time gap after the initial dose of 7 Gy, however, in the second hour the additional loss of efficacy was only 0.5%. These results have been compared with those irradiated continuously over 30, 60 and 120 min (♦). In these studies, the actual data point was for a dose of 14.37 Gy, although a value for 14 Gy (◊) was also obtained by extrapolation from the relative cells survival curve. The two data sets were very comparable for exposures up to 60 min. but for exposures spread over 120 min greater recovery was seen after continuous exposure. Error bars indicate ±SEM.