334 CORNELIUS A. TOBIAS 



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Fig. 4. Photograph of head of rat which received 20,000 rad of deuterons in a 

 small beam 1 X 2 mm in diameter passing laterally through the head. White spot in 

 hair indicates passage of beam. Entry and exit spots are indistinguishable. From 

 Tobias et al. (1954). 



the head of a rat. The hair of the animal turned white where the beam crossed 

 the skin. It is impossible to tell which is the entrance or exit spot on the 

 skin. There are many problems one encounters in actual practical radio- 

 logic procedures. Accurate evaluation of the isodose cur\'es themselves pre- 

 sent a great problem, since the dose varies very quickly with depth. The 

 usual technicjue used is the exposure of a spectroscopical quality of photo- 

 graphic material to the beam in an accurately constructed phantom with 

 identical apertures and rotations as in the actual biologic exposure. One may 

 then evaluate the dose distribution from photographic densitometry. Any 

 such pi-ocedure presupposes knowledge of the dose-effect relationship in 

 photographic film (Tochilin et al, 1955; Welch, 1955). Some corrections 

 are necessary for various factors, e.g., oblique incidence of the beam, spread 

 caused by motion of particles in air, etc. Other useful methods are solid 

 state dosimeters, dosimetry by radioactivation, or the use of a chemical 

 oxidation-reduction system. 



At the Berkeley 184 in. cyclotron, a de\ice is available for exact position- 

 ing, by means of x-ray diagnostic techniques, of any desired part of the head. 

 With the use of a three-dimensional Descartes coordinate system, it is pos- 

 sible to deliver a radiation lesion of arbitrary size to the desired point. It is 

 then possible, by merely changing one single coordinate, to deliver another 

 lesion in the exact bilateral location. Usually the head of the animal that 



