LESIONS WITH ACCELERATED PARTICLES 



333 



20000 



M 



O 



^^ 10000 



o 



o 



ALPHA PARTICLES 

 10.4 ± 0.2 Mev / amu 



0.5 1.0 



Depth, Millimeters 







^■- 



r 



Effect 



1007. 



^Tissue surface A Laminar lesion 



Fig. 3. Depth dose distribution for 40 Mev alpha particles. On upper right is a 

 hypothetical dose-effect relationship. .'Xssumino that effects grt'ater than 50''r are 

 visible under the microscope, the lower cur\e gives profile of the "laminar lesion." 



range in tissue is about 14.5 cm. the width of the Bragg peak is about 4 mm. 

 and the mean lateral spread in the entire range about 3 mm. Figures such as 

 these gi\e an indication of depth and minimal practical lesion size of the 

 particles in Cjuestion. A great deal of experimental work remains as yet to 

 be done to explore the limitations of the technique. It is practical to use 

 knife edge lesions of 1 mm in diameter to a depth of about 4 cm in the 

 brain: as the beam penetrates deeper, however, the lesion becomes less and 

 less definite. 



The cross-sectional area of the beam may be shaped by metal apertures of 

 thickness greater than the range of the beam. Usually, one monitors the 

 beam with ion chambers as it emerges from the accelerator and calculates 

 the ionization as function of depth in tissue from the known stopping power 

 and composition. Experiments with phantoms are possible. 



An illustration of the relati\ e lack of scattering is shown in Fig. 4. Here a 

 beam of 190 Mev deuterons, 1 X 2 mm in cross section, was passed through 



