LESIONS WITH ACCELERATED PARTICLES 335 



is to be exposed is rotated around the point to be irradiated, while a cylin- 

 drical beam constantly radiates the center of rotation. Use of the Bragg 

 ionization peak for such irradiation is also possible, but more elaborate 

 preparations are necessary, since the depth of the range needs to be con- 

 trolled at all times. 



RaDIOBIOLOGIC ClONSIDERATIONS 



The nature of the radiation effect on the adult central nervous system 

 poses great and as yet unsolved problems. The tissue has heterogenous archi- 

 tectm~e: its structme is built for optimum maintenance of its most essential 

 elements, the neurons, and this function is accomplished by an intricate 

 architecture of the \aried cellular components. Proliferating elements of 

 neural tissue are the astrocytes and ulial cells. Xeiuons in the adult do not 

 seem to have mitotic acti\ity. and their functions appear to be mainly in 

 transmission of action potentials and. in some instances, perhaps neurosecre- 

 tion. Our cellular radiobiologic knowledge comes mainly from studies of 

 rapidly proliferatino cells in the coinse of the cell division process, and we 

 know that the maximum killing effect expresses itself mainly during and 

 following the cell di\ision process. 



Since neurons do not divide, it is no wonder that they are thought to be 

 relati\ely radioresistant. However, we do know that the developing embryo 

 ner\ous system is extremely radiosensitixe see R. Ruyh. 1961 i. and 

 studies in progress also point to radiosensitivity of the adult brain: usually, 

 however, a considerable time elapses before the damage is developed to the 

 point where it is pathologically obser\able. The observed change is usually 

 necrosis, that is. disappearance of all cellular elements. Cmrent radiation 

 studies point to the need of understanding the detailed biochemical and 

 cellular processes of necrosis and its initiation. 



There is no definite dose-effect relationship as yet established for nerve 

 tissue. We do know that therapeutic x-ray irradiation of a large part of the 

 human brain may result in late degenerative changes, including demyeliniza- 

 tion. scarring, and necrosis ( Druger ct al., 1954). Lindgren (1958) has 

 collected human material and demonstrated that protracted irradiation 

 schedules are less effecti\ e in causing necrosis than a single dose and that the 

 dose-efTect relationship for protracted dose schedules is similar to that 

 demonstrated by Strandquist (1944) for skin. Arnold et al. (1961) studied 

 radiation effects on brain of primates and found demyelinization. the first 

 observable chronic deleterious effect, one that may be caused by a dose of a 

 few hundred rads. Lindgren's 19581 data on rabbits indicates somewhat 

 higher dose thresholds. At Berkeley many irradiations have been carried out 

 on animals oxer the last few years, usually with the proton or deuteron beam, 



