CARCINOGENESIS BY IONIZING RADIATIONS. 1185 



BRAIN 



This organ is resistant to irradiation and thus far no human or experi- 

 mental brain tumor caused by ionizing radiation is on record. Glia cells 

 of the brain are susceptible to chemical carcinogens, and one might expect 

 the formation of gliomas in brains exposed to large doses of ionizing radia- 

 tion. Yet thus far only sarcomas of the connective tissue and bone 

 tumors have resulted from such exposures (Jentzer, 1937; Lacassagne and 

 Nyka, 1937). 



Wilson et al. (1951) exposed rat embryos to X radiation on the ninth 

 day of gestation and noted the development of discrete tumorlike growths 

 in and around the brain and related these directly to X irradiation. They 

 first appeared on the second day after irradiation and thereafter exhibited 

 varying capacities for growth and differentiation. Some grew for 1 or 2 

 days, then disappeared as a result of dispersal of the cells; others grew 

 rapidly until the fifth or sixth postirradiation day, then became atrophic; 

 and still others continued to grow slowly until the seventh or eighth day, 

 then became static or underwent atrophy and regression. It seems to us 

 that these developmental anomalies should not be designated as 

 neoplasms. 



GENERAL COMMENTS 



Ionizing radiations were the first type of carcinogens thoroughly 

 studied and much of what has been done subsequently with chemical car- 

 cinogens is a duplication of the work with these physical carcinogens. In 

 carcinogenic potency, ionizing radiations are second to none, as indicated 

 by the certainty and frequency with which they induce neoplasms. In 

 variety of neoplasms induced, precision in quantitation, and as a tool in 

 cancer research they match any other carcinogen. Nevertheless, after 

 the hazards of radiations were discovered and adequately controlled, 

 research on this type of carcinogenesis waned until recent developments 

 have made this area again a highly important field of investigation. 



The dose-time relationship after administration of a single dose of a 

 radioisotope was well worked out by Brues (1949) with the use of Sr 89 , 

 which causes neoplasms at the site of deposition. His findings are 

 illustrated in Fig. 18-10. Each quantity of Sr 89 absorbed by bone confers 

 a given probability of bone tumor formation, the tumor development 

 time decreasing and tumor incidence increasing with the dosage. It is 

 not known whether a threshold dose exists for such effects, the limiting 

 factor being the survival time. The daily tumor morbidity will con- 

 stantly increase as long as further irradiation occurs. 



Carcinogenesis by a bone-seeking radioisotope cannot be accurately 

 computed on body-weight basis. The concentration in the area of sus- 

 ceptible cells, rate of elimination from the area, and degree or character of 



