406 JANSSEN ET AL. 



The study with sodium fluorescein was limited to gross observations be- 

 cause it does not locahze microscopically. Parenchymal fluorescence was 

 first seen at 72 hours, and resembled that obser\ed with FLSP, except that 

 the contrast in intensity in grey and white matter was much less distinct. 

 The final stage at which fluorescence in both cerebellum and cerebrum was 

 seen was 14 days after irradiation. 



Discussion 



The foregoing observations supplement in various ways those reported by 

 Malis and his associates (1957 ) dealing with the efTects of protons ( 10 Mev 

 per nucleon) on the cerebral cortex of 2 cats. Although, in terms of energy 

 transfer, comparison of results meets obstacles because of the difficulties 

 they encountered in establishing tissue irradiation dosage, the pseudolaminar 

 cerebral cortical lesion they produced was highly similar to that observed 

 in our animals. 



One of the problems with which we were concerned was whether or not, 

 at a given radiation dosage, alpha particles produced the same pathologic 

 changes as protons in the cerebellum and cerebrum. Although alpha par- 

 ticles and protons with the same energy per nucleon (in the present investi- 

 gation, about 12 Mev per nucleon) have the same range, the stopping 

 power, or linear energy transfer (LET), of the alpha particle is 4 times 

 that of the proton. It was therefore of interest to determine whether the 

 fourfold difference in LET was reflected pathologically. At the 6,000 rad 

 surface dose, the changes produced by these two types of particles were 

 highly similar in the time of appearance of lesions, width of the band of 

 most intense damage (Table II), extent and severity of nerve cell damage in 

 the tissue above the band after a given latent period, and time of appearance 

 of vasodilatation in sections stained by the Pickworth-Lepehne method 

 (Table III). Thus, pathogenically, alpha particles and protons had much 

 the same effect despite the fourfold difference in particle LET. 



In animals exposed to the lowest effective radiation dosage (1,500 rad at 

 the brain surface), a band lesion was evident in the cerebrum at 216 days 

 (Fig. 4), when no changes were observed in the cerebellum (Table III). 

 This was considered as evidence that the cerebral cortex was the more 

 radiovulnerable. At 6,000 rad cerebellar granule cells and nerve cells and 

 glia of the cerebral cortex seemed equally radiovulnerable from the stand- 

 point of the time of appearance of damage, but a zone of damage was evi- 

 dent earlier in the cerebellum (6 hours) than in the cerebrum (42 hours). 

 Invariably the band of nerve cell loss in the cerebellar granular layer was 

 much narrower than that in the cerebral cortex (Table II). Whether this 

 means that nerve cells of the cerebral cortex were more radiovulnerable than 



