ULTRAVIOLET RADIATION AND CANCER 



543 



tumor cells. If it is assumed that the number of tumor cells so produced 

 is small and their inherent rate of proliferation low, it is conceivable that 

 no tumor would be detected, within the lifetime of the mouse, as a result 

 of a single or a small number of doses of ultraviolet radiation. On the 

 other hand, if the dose is repeated many times at appropriate intervals, it 

 might be that enough tumor cells would be produced by mutation and })y 

 proliferation of the mutants to result in an observable tumor. Such a 

 concept fits qualitatively with the experimental finding that a single dose 

 of ultraviolet radiation does not produce a tumor, but repeated doses do. 

 When this concept is examined quantitatively, however, grave difficulties 

 arise, as a simple arithmetical calculation shows. This is illustrated in 

 Table 14-3, where it is assumed that each tumor cell divides every 6 days; 



Table 14-3. Grov/th of a Tumor: Assuming That One Tumor Cell 



Originates Each Day by Mutation and That All Tumor 



Cells Divide Every Sixth Day 



this corresponds to the average growth rate for these tumors after they 

 have reached measurable size. It was also assumed that one dose of 

 ultraviolet radiation is applied each day, and that this dose causes one 

 tissue cell to mutate to a tumor cell. Within a relatively short time after 

 the first dose the contribution of tumor cells by mutation becomes negli- 

 gible compared to the number contributed by proliferation of those tumor 

 cells already present. Substitution of other reasonable values for the 

 ones used in the example leads to essentially the same result. It is obvi- 

 ous that, if the development of a tumor depended on the accumulation of 

 tumor cells by mutation and proliferation under conditions such as those 

 suggested, after a very short time the application of further doses of radi- 

 ation would have no detectable effect on the rate of development of 

 tumors. Yet the application of doses of radiation as late as, say, 100 

 days after the first dose may ha\e a marked effect on the rate of develop- 

 ment of the tumors, as is illustrated in Fig. 14-8. It can only be con- 

 cluded that the rate of tumor growth is not constant throughout the 



