CARCINOGENESIS BY IONIZING RADIATIONS 1161 



exposure to fast neutrons was 22 to 32 per cent in groups of mice exposed 

 to 26 and 90 N as compared to 14 per cent in controls. The variations in 

 relation to dose seemed not significant. In parallel experiments they 

 studied the leukemia incidence following a single dose of 7 radiation and 

 found this to be 67 per cent in mice exposed to 500 r and 64 per cent in 

 mice exposed to 700 r. 



The effects on neoplasia incidence of slow neutron exposure are being 

 studied currently in the biological tunnel of the Oak Ridge reactor. 

 Analyses about one year after exposure indicated a five- to tenfold 

 increase in leukemias in the dose ranges at or moderately below the LD 50 

 dose. When compared on the basis of LD 50 , no preferential effect was 

 noted between slow neutrons and X radiation. 



The chronic effects of neutrons from cyclotrons were studied by a few 

 investigators, but unfortunately either the animals were not observed 

 long enough or no distinction was made between leukemias and leukemoid 

 reactions. Leitch (1947) noted an increased incidence of mammary 

 tumors in rats exposed to cyclotron neutrons. 



In exploring the possible role of cosmic rays in carcinogenesis, Figge 

 (1947, 1949) claimed that the potency of chemical carcinogens may be 

 altered by slight variations in the intensity of penetrating environmental 

 radiations. No data are available to indicate that cosmic radiation is 

 carcinogenic. No such effect was found by Franks and Meek (1950) in a 

 carefully controlled experiment. 



ORGAN SUSCEPTIBILITY TO TUMOR FORMATION 



It is probable that all cells capable of multiplication are susceptible to 

 the induction of neoplasia by irradiation. The incidence and sites of 

 neoplasia depend on the amount of energy absorbed by the cells, the 

 functional state and radiosensitivity of the cells, and other factors. 

 There are ample observations to indicate that hemopoietic cells, notably 

 lymphocytes, are highly radiosensitive, while macrophages (histiocytes, 

 reticuloendothelial cells) are highly resistant. Specialized cells, as osteo- 

 blasts, are highly radiosensitive; muscle, liver, and kidney cells are 

 more resistant. The most radiosensitive organ appears to be the 

 mouse ovary. 



Studies published up to 1942 on the effects of irradiations on normal 

 tissues have been reviewed in a series of papers by Shields Warren and 

 associates (1942-1943) and the histological changes also by Bloom (1948). 

 It is difficult to compare the relative sensitivity of different normal cells 

 on the basis of total-body irradiation, since they are not in a comparable 

 state of the reproductive cycle or under identical physiological and 

 environmental influences. The differences in response of different types 

 of cells and of different cells of the same type to similar ionizing radiations 

 are truly remarkable (Lacassagne, 1936). As he phrased it: 



