364 



MALIS, ROSE, KRUGER AND BAKER 



Fig. 5. Scheme of the irradiation arrangement. The telescope, which indicates the 

 position of the target area in relation to the beam and the distance of the target from 

 the ion chamber, is shown in the position it occupies during irradiaton. An absorber 

 (often used to shorten the range of particles in tissue) is advanced in the absorber 

 rack to a distance of about 12 mm from the target to minimize the effects of scatter. 

 The absorber does not touch the target in order not to compromise the sterility of 

 the exposed cortex. The animal is positioned in relation to the beam in the milling 

 machine. 



rise in ionization near the end of the rans,e makes possible the production of 

 a laminar lesion. Considering that one mil thick aluminum foil is nearly 

 ecjuivalent in its stopping power to 50 fx of brain, one can estimate the prob- 

 able width of laminar destruction from the ionization curve. There is a 

 reasonable, thotigh not strict, relationship between the expected and meas- 

 ured thicknesses of the laminar lesions for light and moderate radiation 

 doses. Since the zone of maximal ionization occurs near the end of the beam 

 path, it is readily possible to shift this zone, and hence the locus of the lami- 

 nar lesion, if the range of particles is shortened in tissue by introducing 

 energy-degrading materials into the beam path. It follows, from considera- 

 tion of the stopping power of brain and aluminum, that introduction of one 

 mil thick aluminiun foil elevates the zone of maximal ionization in the brain 

 by about 50 /x. 



Several difficulties arise in the determination of the radiation dose. The 

 first group of problems pertains to the determination of the actual number 

 of particles which strike the target in a given irradiation arrangement. 

 Further difficulties occiu' when one wishes to express the radiation dose in 

 a manner which has biologic meaning, once the number of particles bom- 

 barding the target is known with reasonable accuracy. 



