MULTITARGET ANALYSIS 



115 



10 20 30 



X-ray dose (kr) 



40 



Fig. 54. Bacterial survival after various x-ray doses. The age of the culture in 

 hours is listed with the survival curve for bacteria of that age. (From Stapleton, 

 J . Bacteriology, 20, 357, 1955; courtesy the author and Williams and Wilkins Co., 

 Baltimore, Md.) 



we obtain a multitarget curve as a result of an experiment, we simply 

 draw a line of the same slope so as to pass through the 100% survival 

 point, and we can then calculate the value of n. In addition, from the 

 mononucleate survival curve, we can find the 37% survival dose, and 

 then obtain the radiosensitive volume as before. 



The value of n can also be found by extrapolating the straight-line 

 portion of the survival curve back to the zero dose line; the intercept is 

 the nuclear number itself. That this is true can be seen from the last 

 equation: log (N/N ) = log n — D. 



If we set D equal to zero, we obtain log (N/N ) = log n. This way of 

 finding n is illustrated in Fig. 53 by extrapolating the n = 2 and n = 10 

 curves back to the zero dose line. 



The student should be warned that there are serious possible com- 

 plications which can arise in a radiobiological experiment. For example, 

 there are cases of irradiation of polyploid yeasts in which the target 



