Abstract
Survival curves have been obtained for a tetraploid strain of the Ehrlich ascites tumor after irradiation with 200 kV X-rays and with fast neutrons in the presence and in the absence of oxygen. The oxygen-enhancement ratio for X-rays is 3 · 1 and for fast neutrons 1 · 8. The RBE of fast neutrons relative to 200 kV X-rays for loss of reproductive integrity is 2 · 1 in oxygen and 3 · 6 in nitrogen. The radiosensitivity of a hyper-diploid strain as defined by the slope of the exponential part of the survival curves is the same when the irradiations are carried out in oxygen. Splitting of the total dose into two fractions given at various times resulted in an increase of the surviving fraction of the cells compared to those which received the total dose in one single shot. The rate of recovery appeared to be similar for the two radiations used, and recovery was at its maximum 5 to 7 hours after the first dose.
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