140 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1945 



harnessing of atomic energy resulting from the artificial disintegra- 

 tion or transmutation of elements could indeed transform the world 

 in a manner beyond the dreams of the old alchemists. Against the 

 background of world war and the sinister possibilities that can be con- 

 jured up, it is to be hoped that the future results of "atom smashing" 

 will not be merely destructive but rather for greater social integration 

 and the increased welfare of mankind (14). 



THE CYCLOTRON AS A MEDICAL INSTRUMENT 



The physics of the cyclotron have been described in other papers 

 in this Journal and elsewhere (9, 15). It was first designed for use 

 in the domain of atomic physics, but it has provided medicine and 

 biology with two new tools for research, viz, a beam of very pene- 

 trating uncharged particles or neutrons 5 and an array of artifi- 

 cially radioactive substances which can be used in two different 

 ways. They can be administered in quantities so small that their 

 presence can be detected only by sensitive physical apparatus, and 

 in this high dilution no biological effects of the substance can be 

 observed with the means at present available. The progress of the 

 radioactive atom through the organs of plant, animal, or man can, 

 however, be followed by physical detectors, and yields information 

 otherwise unobtainable. Illustrations of the use of these "tagged 

 atoms" or tracers in biological and clinical research will be given 

 later. 



In larger quantity, radioactive substances can be administered so 

 that the radiation they emit is used in some localized region of 

 the body to produce definite biological changes — usually destructive 

 ones such as the elimination of unwanted cells. The problem in this 

 case is to localize the radioactive substance in the particular region 

 required. This is not always so difficult as it might seem at first 

 sight. Selective absorption of certain substances in particular 

 organs has been familiar for a long time. And since virtually all 

 the known elements can now be made artificially radioactive, it is 

 possible to select the most suitable element for the purpose required 

 and then proceed to make a radioactive compound in the composi- 

 tion of which it is included. The length of life of the artificial 

 product is the limiting factor and that cannot be predetermined. 

 This therapeutic administraton of radioactive substances is a more 

 difficult field of research than metabolic studies with tracer sub- 

 stances, and progress must of necessity be slower and less spectacular. 

 But already it is possible in certain instances (see later) to achieve 

 as much by the internal administration of a radioactive element in 



5 The cyclotron is the most efficient but not, of course, the only means by which neutrons 

 can be produced. 



