922 RADIATION BIOLOGY 



This diversity of action increases the probability of nonspecificity, and 

 so we observe that no single response is peculiarly specific for radiation 

 injury, while many agents (such as urethane, nitrogen mustard, or 

 benzol) are capable of mimicking radiation effects. Thus many of the 

 effects of nitrogen mustard on enzymes and cells are indistinguishable 

 from those of radiation and point to similar chemical processes (DeLong, 

 1950). Yet radiomimesis has its limitations in that no single agent 

 duplicates all the radiation-induced reactions. In part at least this 

 difference is one of spatial distribution; owing to natural barriers to the 

 diffusion of a chemical agent from its point of entry and to conditions of 

 its degradation, it would be almost impossible for such a radiomimetic 

 agent to attain the uniform distribution of penetrating radiation. 



Depending upon the character of the radiation and the manner of 

 exposure, a variable period intervenes between irradiation and the various 

 observed effects. This interval may be a matter of minutes for suppres- 

 sion of cell division, hours for lymphopenia, days for neutropenia and 

 hemorrhage, weeks for anemia, or months for induction of tumors. Some 

 of these intervals may be correlated with the life span, mitotic rate, and 

 metabolic activity of the cells concerned with the effect. Many critical 

 events, presumably chemical in nature, must occur during this "latent" 

 period. 



RADIATION QUALITY AND QUANTITY 



As an environmental change or stimulus, radiation in general conforms 

 to the familiar concepts of threshold, summation, intensity-duration, 

 and adaptation. The energy required to produce different biological 

 effects varies considerably. Retardation of growth of Phy corny ces has 

 been observed after only 0.001 r, while 1 r is sufficient to inhibit the 

 activity of aged preparations of adenosinetriphosphatase (Forssberg, 

 1943; Barron, Dickman, et al., 1949). Hematologic changes in mammals 

 are seen with dosages of 25 to 50 r. Yet other effects such as the inhibi- 

 tion of contraction of striated muscle or immediate mammalian death 

 may require many thousands of roentgens. 



In general for any particular radiobiological response, there are limits 

 within which the quantity of radiation will influence the effect in terms of 

 latent period, severity, and recovery. In the mouse, total-body gamma- 

 ray dosages of 140,000 r delivered in 20 min lead to immediate death 

 (Henshaw, Snider, et al, 1946). With the same dose rate, 70,000 r will 

 kill in 1 to 5 hours, and 35,000 r in 7 to 62 hours. On the other hand, 

 after dosages between 3500 and 14,000 r, there is no difference in survival 

 time; all mice live 4 to 5 days. When the quantity of radiation is 

 decreased still further to the minimum LDioo and below, survival time 

 again increases, most deaths occurring at about two weeks. Similar 

 findings have been observed with X rays and point to a distinct separa- 



