EFFECTIVENESS OF X-RAY WAVE-LENGTHS 461 



action but rather to the fact that the intensity of the beam is very low. 

 When X-rays are deUvered at comparable intensities, the tissue reactions 

 are the same as to radium. The rate at which the radiation is delivered 

 is the important factor. There is no evidence that short rays induce 

 results which are biologically different from those brought about by 

 longer rays, or that they possess potencies not shared by the latter. 



Why, then, should beams of low intensity show elective action? 

 The answer lies in the biological properties of the cells rather than in 

 the physical nature of the radiations. When a beam of high intensity 

 is absorbed in a tissue, the very extensive ionization which ensues initiates 

 destructive changes in all of the cells whether they are naturally resistant 

 or sensitive, and none escapes injury. But the slight damage done by a 

 beam of low intensity can be repaired by the resistant cells as rapidly as 

 it is inflicted, so that at the end of even a long exposure they appear to 

 be unharmed. The reverse is true for sensitive cells and they soon 

 succumb. 



The conclusion that the nature of the reaction is not influenced by the 

 wave-length of the incident beam is supported by physical considerations. 

 When X- and gamma rays are absorbed, a primary electron is ejected from 

 the absorbing atom. This electron, whose velocity depends on the wave- 

 length of the incident beam, collides with atoms along its path, setting 

 free hundreds or even thousands of secondary electrons from them, and 

 continues so to do until it has expended all of its energy and comes to rest. 

 The extensive ionization produced in this way is the starting point of a 

 series of biological reactions which may lead to the injury or death of the 

 cells. If then the same number of secondary electrons are produced by 

 both hard and soft rays; if, in other words, the same amount of ionization 

 occurs, we should expect the biological reaction to be the same also. 

 And when the amount of ionization is greater in one case than another, 

 we should expect a difference only in the magnitude of the reaction and 

 not in its nature. 



QUANTITATIVE EFFECTS 



Whether equal quantitative effects are produced by equal doses of 

 radiation having widely different wave-lengths is a question which has 

 not yet been finally answered. Its solution involves two problems; 

 first, the accurate measurement of the dose, and second, the accurate 

 measurement of the biological effects which it produces. Methods for 

 determining dosage are discussed elsewhere. At the present time the 

 general opinion is that X-ray dosage can be measured v;ith reasonable 

 accuracy when the rays are produced at voltages lying within the range 

 commonly used in therapy. Difficulties arise w^hen these limits are 

 exceeded, especially on the side of very high voltages. Indeed, measure- 

 ments of X-rays produced at 500 kv. and more are only approximate, 



