CHAPTER O 



The Radiobiological Importance of Linear Energy Transfer 



Raymond E. Zirkle 

 Institute of Radiobiology and Biophysics, University of Chicago 



Introduction. Experimental variation of linear energy transfer. Estimation of dose: 

 X and gamma rays — Beta rays — Fast neutrons — Alpha particles. Results of the avail- 

 able investigations: Explanation of Table 6-1. Discussion of results: Cases of no change 

 in effectiveness — Coses of decrease in effectiveness with increase in linear energy transfer — 

 Cases of increase in effectiveness with increase in linear energy transfer — Cases of effective- 

 ness passing through a maximum — Cases in which dose-effect curves change shape — 

 Influence of linear energy transfer and the nature of the irradiated object — Additivity of 

 radiations of different linear energy transfer — Theoretical implications of the observed 

 influence of linear energy transfer. References. 



INTRODUCTION 



When a cell or organism is exposed to any high-energy (ionizing) radia- 

 tion, ions and excited molecules are not formed singly and at random 

 throughout the cell or organism, but are localized in tracks of high-energy 

 particles. These tracks, so clearly demonstrated in gases by means of the 

 Wilson cloud chamber, are produced by all speedy charged particles (elec- 

 trons, mesons, atomic nuclei). The total volume of the tracks formed, 

 even by very injurious doses of radiation, is a small fraction of the total 

 volume of the cell or organism. 



So far as is known, the properties of the ions which are produced along 

 an ionization track are independent of the properties of the high-energy 

 particle. This is also true of the excited molecules, i.e., those activated 

 to states lower than ionization. On the other hand, the hnear spacing 

 of the ion pairs and excited molecules along the tracks varies widely with 

 the velocity and charge of the ionizing particles. 



The linear spacing of the ions is customarily discussed in terms of its 

 reciprocal, the specific ionization or linear ion density, i.e. the number of 

 ion pairs formed per unit length of track. In gases this quantity can be 

 determined by straightforward physical methods. In tissue, however, 

 it must be obtained by dividing the energy released per unit length of the 

 particle's track by the tissue's energy gain per ion pair produced. This 

 latter quantity is frequently assumed to be roughly the same as that 

 observed in air (35 ev per ion pair for slow particles, 32.5 for others) but 



315 



