PATHOLOGIC CHANGES IN BRAIN CAUSED BY ALPHA PARTICLES 409 



exposed. The maximal tissue dosage was approximately 5 times that at the 

 brain surface. 



No essential differences were observed in the alterations produced by- 

 alpha particles and those produced by protons. 



The width of the zone of damage in the cortex of both the cerebelkan and 

 cerebrum corresponded to the amount of energy gi\en off by the particles as 

 expressed by the Bragg cinve. 



Granule and Purkinje cells of the cerebellum and nerve cells and glia in 

 the cerebral cortex were highly radio\ulnerable. The greater width of 

 the band of damage in the cerebrum than in the cerebellum and the observa- 

 tion that at the lowest effective radiation dose (1,500 rad at the brain 

 surface) a band of nerve cell loss was found in the cerebrum but not in the 

 cerebellum suggested that the cerebral cortex was the more radiovulnerable. 

 From other standpoints. howe\er, the cerebellar granular layer seemed the 

 most radiovulnerable. 



Nerve cell and glial damage was incurred before circulatory or perme- 

 ability disturbances were evident, strongly suggesting direct irradiation of 

 cellular elements as the primary factor in pathogenesis. At subsccjuent stages 

 circulatory changes and increased vascular permeability contributed to the 

 extent of the lesions. 



No lesions were encountered after as long as 7 months at surface doses of 

 50, 250, and 750 rad. The earliest times at which damage to cellular elements 

 was obserxed following exposure at other siuface doses were as follows: at 

 1 ,500 rad. 7 months ( in cerebrum alone ) ; at 3,000 rad. 20 days; at 6,000 rad, 

 6 hours; and at 12,000 rad, 3 hours. 



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