356 ANDERSSON, LARSSON, LEKSELL, MAIR, REXED AND SOURANDER 

 Discussion 



The proton beam as an agent for production of lesions in the central 

 nervous system has the advantage that the particles of the beam proceed 

 along practically straight tracks and undergo little scattering. The ionizing 

 effect is thereby restricted, and it should be theoretically possible, therefore, 

 to cause a sharply defined lesion. 



The lesions in nervous tissue caused by high energy protons v^as that of 

 an acute necrosis and similar to that described by Arnold and co-workers 

 (1954) after irradiation of the brain of the monkey with high energy roent- 

 gen radiation. In our experiments on the spinal cord, as well as in those by 

 Arnold and co-workers (1954) on the brain of the monkey and by Larsson 

 (1960) on the brain of the rat, it appeared that the latent period for radi- 

 ation necrosis depends on the dose of radiation. A dose of 20 krad with a 

 1.5 mm-broad proton beam gave a well defined lesion after 9 days. With 

 larger and smaller doses, the latent period was shortened or prolonged, 

 respectively. A radical increase of the dose above 20 krad, considered opti- 

 mum dose, induced in the spinal cord, as well as in the brain, a more 

 widespread tissue reaction. This observation stresses the importance of select- 

 ing the dose carefully when planning any surgical procedure. 



The breadth of the beam proved to be a factor of great importance. In 

 the experiments on the spinal cord a 10 mm beam not only produced 

 necrosis of about the same breadth as the beam, but also produced a lesion 

 considerably more quickly and of a histologically more severe type than a 

 thinner 1.5 mm beam. With the thin beam, only slight vascular damage 

 and minimal hemorrhages were seen. The same dose and a broad beam 

 caused, even after 1 day, signs of vascular damage, and after 3 days, plasma 

 exudation and numerous hemorrhages, especially in the gray substance. 

 It is assumed that the peculiar type of \ascular arrangement in the spinal 

 cord and the poor development of collaterals in its thoracic part favored 

 the appearance of large hemorrhages when a considerably large portion of 

 the cord was irradiated. 



In experiments on the tolerance of mouse brain tissue to high energy 

 deuterons, Zeman and co-workers (1959) fomid that the dose required to 

 produce a threshold lesion in mouse brain increased from 30 krad with a 

 beam 1 mm broad to 1.1 X 10'' rad with a beam 0.025 mm in diameter. 

 According to these authors, a possible explanation would be that the micro- 

 beams cause a predominantly direct radiation injury, whereas the broad 

 beams produce additional indirect effects, e.g., vascular disturbances. 

 Arnold and co-workers using 10 or 25-mm-broad beams of high energy 

 roentgen rays did not find evidence that the lesions produced by broad 

 beams were secondary to vascular occlusion. In our experiments the narrow- 



