X-IRRADIATION STUDIES ON THE MAMMALIAN RETINA 545 



x-irradiation on the electroretinocjram — a "mass response"' of the retina — • 

 were evaluated in terms of the dose delivered to the anterior rim of the 

 retina, the posterior pole, and an imaginary circle dividing the retina into an 

 anterior half and a posterior half. The dose delivered to this imaginary circle 

 (called the medium retinal dose) served as the principal reference. The 

 minimal doses for visual cell death were determined histologically, usually 

 after 10 to 14 days. The rate at which the medium retinal dose was delivered 

 to the eye closest to the beam was, in rads per minute, 174 for 100 kvp, 482 

 for 250 kvp, and 302 for 2,000 kvp. Usually, the eyes were irradiated once; 

 and. usually, results refer to irradiation at 250 or 2,000 kvp. 



The electroretinogram (ERG) was recorded continuously during irradia- 

 tion in response to brief flashes of light throughout irradiation and at pre- 

 determined times after irradiation. 



Histologic Effects 



The typical appearance of the rabbit retina after a dose which produced 

 visual cell death is illustrated in Fig. 2. One medium retinal dose of 5.300 

 rads had been delivered in 1 1 minutes. Ten days after irradiation, the visual 

 cells have disappeared. The pigment epithelium is presei-\ed, and ganglion 

 and bipolar cells are not measurably decreased in nimiber. 



Using 2,000 kvp x-rays, death of the rabbit's visual cells resulted when, 

 on the average, 4,000 rads had been delivered to the area. As described by 

 Baily and Noell I 1958), the efficiency of radiation in producing the retinal 

 changes seems to increase with decreasing linear energy transfer. The 

 average lethal cell dose for 100 kvp irradiation was almost 8.000 rads. 



Pyknosis was the first sign of visual cell death, appearing in 24-48 hours, 

 and was invariably followed within 1 or 2 days by chromatolysis and cell 

 disintegration. Ganglion cells and bipolar cells withstood a dose twice that 

 which killed the visual cells. 



In the monkey x-irradiation destroyed primarily the rod cells, thus dif- 

 ferentiating between rod and cone cells (Cibis et ai, 1955). The rabbit 

 retina is mainly one of rod cells, and its cone cells seem to differ only 

 slightly in susceptibility. 



Visual cells which survived a single dose slightly below the cytocidal dose 

 showed characteristic degenerative changes of their rods, with thinning and 

 partial or complete disintegration of the outer segments and swelling of the 

 inner segments ( Fig. 3 ) . Lesions of this kind were found after a single dose 

 of 250 or 2.000 kvp irradiation, but only near the area of visual cell death. 

 Widespread degenerative changes without cell death were evident after 

 repeated x-irradiation with small doses (see Fractionation of Dose). In these 

 experiments, the ERG became severely afTected, as if a large fraction of the 

 visual cell population had died. 



