NEUTRON EFFECTS ON ANIMALS 41 



sensitive to neutron irradiation than the females. However, since only 

 eleven rats were involved in this experiment and none of the other experi- 

 ments showed a similar variation in sensitivity dependent upon sex, addi- 

 tional experimental data under comparable conditions of irradiation are 

 necessary before the significance of these results can be determined. It 

 should be emphasized that nothing was observed during the course of this 

 experiment that might account for the apparent sex difference. 



Of primary significance in this experiment was the appearance of a tumor 

 in one of the female rats (see rat IX-7, Table II), since this was the first 

 indication of the initiation of tumor growth by neutrons. Microscopically, 

 it was found that one other animal in this experiment (rat IX-8, Table II) 

 showed evidence of extensive malignant growth in the abdominal organs. 

 This latter case was the only instance of tumor formation in Avhich gross 

 evidence thereof was not apparent and also in which the abdominal organs 

 were involved. The initiation of tumors by neutron irradiation will be 

 discussed further in connection with the data for those animals receiving 

 repeated irradiation with 1.8 n of neutrons. 



Repeated Doses of 1 .8 n. In this group there were three different experi- 

 ments in which rats received 1.8 n of neutrons, six days a week, for several 

 months. Of greatest significance is the fact that although there were no 

 changes in the peripheral blood, neutron irradiation did produce a definite 

 reduction in the rate of growth and furthermore numerous tumors de- 

 veloped in the irradiated rats. The change in the growth rate was only 

 apparent on comparison with the corresponding non-irradiated rats. The 

 rats throughout the experiments seemed to be in quite good condition 

 although some roughness of the fur was apparent. This growth reaction 

 to repeated low doses of neutrons is of little value as a criterion of injury 

 when applied to the safety of radiologists since comparative data would 

 not be available. 



Since no previous report on the initiation of spontaneous tumors by 

 neutrons has been found in the literature, further discussion of this phase 

 of the results is desirable. First it should be noted that no spontaneous 

 tumor formation has been observed in the Brooklyn Strain of rats as main- 

 tained in this laboratory so that these results must be considered as either 

 direct or indirect action of neutron irradiation. Tumor formation was 

 first observed following twelve 10-n doses where it was preceded by definite 

 and severe hematological changes indicative of neutron injury. However, 

 after repeated doses of 1.8 n, there was no definite indication of neutron 

 injury prior to the appearance of small subcutaneous tumors. These 

 tumors were palpable from 170 to 239 days from the start of irradiation 

 after from 14G to 205 doses of 1.8 n (accumulative doses of 263 n to 309 n). 

 However, following twelve 10-n doses, the rats developed tumors after an 



