788 RADIATION BIOLOGY 



Lasnitski (1948) determined the relative efficiencies of /? and X rays 

 in lowering the mitotic count in the chick fibroblast culture. She found 

 that, although the over-all depression of mitotic activity during the 

 24-hour period following treatment and the maximum reduction of 

 mitosis was comparable for the two radiations, mitotic activity fell off 

 more abruptly and during recovery rose more rapidly in the /?- than in the 

 X-irradiated material (Fig. 11-15). This illustrates very clearly the 

 danger of interpreting the extent of radiation-induced mitotic depression 

 on the basis of mitotic counts made at the end of a single time interval 

 after treatment. 



Dosage Rate. In the great majority of studies in the field of radiation 

 biology treatment administered is expressed in terms of dose, with little 

 emphasis placed on the time-intensity relationship. The dose, however, 

 represents only the total quantity of radiant energy dissipated in a tissue. 

 It tells us nothing of the intensity of the radiation or of the time of 

 exposure, and these may be quite important in determining the effect 

 produced. Excellent discussions of these factors in biological research 

 are presented by Lea (1938a, 1946) and Gray et at. (1944). 



The time-intensity factor is important, and at the same time peculiarly 

 difficult to deal w T ith experimentally for several reasons. First, if 

 recovery of the cell from radiation effects occurs at all, it begins immedi- 

 ately after the first effects are produced and continues during treatment 

 (Canti and Donaldson, 1926; Henshaw et al., 1933); therefore the bio- 

 logical effect as determined at the end of a long treatment period, during 

 which recovery from the earlier effects produced has occurred, will be 

 less than that obtained after a corresponding dose delivered at high 

 intensity in a shorter period. Second, a certain detectable biological 

 effect may depend on interaction between the products of two or more 

 primary effects that are subject to recovery during the treatment period— 

 perhaps at the molecular level or perhaps at the microscopic level, e.g., 

 broken chromosomes which by interaction (fusion) give rise to transloca- 

 tions. If these are subject to recovery during the treatment period, 

 small doses will produce little or no effect, whether delivered in a short 

 period at high intensity or in a long period at low intensity, because the 

 primary changes will be so few and far between on the average that little 

 or no interaction will take place. At successively larger doses, however, 

 interaction will have a successively better opportunity of occurring and 

 the observed effect will increase theoretically as the square of the dose- 

 actually it is usually less than this because of the recovery that takes 

 place during treatment. Third, since the living cell is dynamic and not 

 static and since visible biological effects cannot be detected immediately 

 after treatment but only at some later phase of a cell's physiological and 

 morphological state, and then may be manifest maximally for only a brief 

 period, results will often depend on the time after treatment at which the 



