420 CLEMENTE AND RICHARDSON 



surrounding zones (Courville and Myers, 1958). For the most part, authors 

 reporting this process describe vascular changes of such a nature as to ac- 

 count for the necrosis either on a hemorrhagic or ischemic basis (Marburg 

 et al., 1945; Pennybacker and Russell, 1948). Analysis of vascular reactions 

 to irradiation in other tissues revealed that following irradiation it takes 

 about a month to produce an ischemic ulcer in the skin(Borak, 1942a). Once 

 a segment of skin has been irradiated, it continues to be more susceptible to 

 noxious influences such as heat, cold, infection and trauma. Dynes and 

 Smedal (1960) report a case of radiation necrosis precipitated by trauma, 

 and other authors suspect a progressive deterioration of blood vessels follow- 

 ing high doses of radiation (Russell et al., 1949; Berg and Lindgren, 1957; 

 Bailey f^ al, 1957). 



More unique experiments have been reported in which brain destruction 

 does not seem to be a direct result of vascular damage. Malis et al. (1957) 

 reported the production of laminar lesions in the cerebral cortex with a 

 minimum of 15,000 rads peak dose. This is 2 to 3 times the minimum dose 

 necessary to produce a lesion when a large volume of brain is irradiated with 

 a uniform dose. Even under these conditions, capillaries appeared to dilate 

 early, and occasionally extravasation of blood elements occurred. Older 

 "line lesions" showed thickening of capillaries (J. E. Rose et al., 1960). 

 Zeman et al. (1959) irradiated mouse brains with beams of deuterons of 

 varying diameters. The beam diameters ranged from 1 mm to 25 jx. They 

 found that 15,000 rad were below a minimal effective dose for a 1 mm 

 diameter beam, 75,000 rad were below the minimal efTective dose for a. 75 fx 

 beam and 5.5 X 10' rad were not minimally effective with a 25 /x beam. 

 With the smaller beams, little or no involvement of the vasculature occurred, 

 presumably because little or no vasculature was in the field; hence, under 

 these conditions, the production of a lesion seemed dependent on the "direct 

 killing" of brain parenchyma. 



In 28-day-old rats, the effects of alpha particle radiation on the vascula- 

 ture and parenchyma of the central nervous system were studied. The radia- 

 tion was delivered to the lower bulbar or upper cervical spinal cord region 

 from the 184 in. cyclotron at Berkeley, California. The beam was directed 

 through 1X5 mm slit aperture and delivered at approximately 2,000 rad 

 per min with doses of 10,000 or 12,500 rad. In other animals, the beam was 

 directed through a 3 X 4 mm aperture over the cerebellum. Animals were 

 sacrificed at varying periods up to 4^2 months after irradiation. Often a 

 rather large cyst developed in the central nervous system at the radiation 

 site (Fig. 5A), but perhaps the most consistent observation was thickening, 

 hyalinization, and often occlusion of the blood vessels. Both surface vessels 

 (Figs. 5B and 5C) and deep vessels (Figs. 5D-F) were affected. In certain 

 areas (Figs. 6A and 6B), virtually all smaller vessels were hyalinized and 



