372 D. C. VAN DYKE, P. JANSSEN AND C. A. TOBIAS 



opened and the head immersed in 10'"r formalin. The brain was removed 

 from the skull the followins: day and placed in fresh formalin. When 

 thoroughly fixed, the irradiated area was blocked out, dehydrated in graded 

 alcohols, and embedded in paraffin. Sections were cut at 6 ^u, and routinely 

 stained with hematoxylin-eosin, \an Gieson, Kr thionin solution for Nissl 

 substance, and Palmgren's silver stain for fibers. 



Throughout this study, 900 Mev alpha particles from the 184 in. cyclotron 

 were used. All effects described resulted from a single dose given at a dose 

 rate of 2,500 rad per min. 



The beam was monitored with a parallel plate ionization chamber with 

 aluminum foil electrodes and nitrogen atmosphere (Tobias et al, 1952; 

 Birge, ct al, 1956). For absolute standardization, this ion chamber was cali- 

 brated against a Faraday cage, which was used to measure the particle ffux 

 in the beam. The distribution of beam intensity after the beam passed through 

 the aperture was calibrated with photographic emulsion dosimetry. By this 

 technicjue, dose of beam measured by the monitor ion chamber was allowed 

 to pass through the aperture and fall on a photographic film exposed in a 

 phantom to correspond with the position of the irradiated animal's head. 

 Densimetry of the photographic film yielded information for dose distribu- 

 tion. Dose values are in rads and refer to peak dose at the central plane of 

 the knife-edge lesion. 



Results 



The minimum dose which would produce changes and the time of onset 

 of demonstrable radiation changes in the brains of rats were studied by 

 giving single doses from 5,000 to 26,000 rads and autopsying the rats at 

 postirradiation intervals from 6 hours to 100 days. The results were evaluated 

 by the presence of fluorescein staining and by morphologic changes seen with 

 light microscopy. Figure 3 summarizes the results obtained in rats, using 

 fluorescein staining as the criterion of damage. The time of onset and inten- 

 sity of fluorescein staining is different for diff"erent doses of irradiation. At 

 the lowest doses of 5,000 and 6,000 rad, no staining occurs until several 

 weeks after irradiation, the intensity of staining is low, and the eflfect appears 

 to be transient. Following doses of 26,000 rad, there was definite staining 24 

 hours after irradiation and liquifaction of the irradiated zone by the 3rd 

 postirradiation day. Doses of irradiation differing by 20'"r betw^een these two 

 extremes can be clearly separated both as to time of onset and intensity of 

 staining. Figure 4 is a log-log plot of the time of first appearance of fluo- 

 rescence as a function of dose and serves to illustrate the definite relationship 

 between dose and time of onset of recognizable changes (staining with 

 fluorescein) . 



