332 



CORNELIUS A. TOBIAS 



5x10" 



• 10^ 



10 100 1000 



Energy, Mev / nucleon 



Fig. 2. Rate of energy transfer from various particles to water as function of the 

 energy per nucleon. Rate of energy loss in soft tissue is within 2% of this value. 



An actual beam of nearly monoenergetic particles ionizes along the so- 

 called Bragg ionization curve, usually arrived at experimentally. This rela- 

 tionship takes into account all phenomena of scattering and straggling. 

 Figure 3 shows the Bragg ionization curve as measured for 40 Mev alpha 

 particles for the purpose of the work reported at this conference with 

 Haymaker ct al. Such a beam can cause laminar neurological lesions. In 

 order to understand the way such lesions originate, Fig. 3 also contains a 

 hypothetical dose-efTect relationship of the multiple hit type. It is easy to 

 understand that pathologically visible lesions occur only in regions where 

 the dose, as represented by the Bragg ionization curve, exceeds a certain 

 threshold value. 



Experimentally the ratio {\/^:^R)/R is between 1/40 to 1/60 in the 

 energy range of 10 Mev/nucleon to 200 Mev/nucleon. The lateral spread of 

 the beam is such that it is possible to maintain a lateral variation of {\/r-)/R 

 somewhat less than the magnitude of the range \ariation (\/A^/?)/i?. For 

 an alpha particle beam of 10 Mev/nucleon the range is about 1300 fx, 

 in tissue; the width of the Bragg peak is about 100 /x and the lateral spread 

 of the beam in 1300 /x distance is about 10/x; for 190 Mev deuterons the 



