MEDICAL USES OF THE CYCLOTRON — SPEAR 141 



the patient's own home as could previously be done by a series of 

 exposures to X-radiation which is a much more exhausting experi- 

 ence for the patient and requires either in-patient accommodation 

 or at least frequent visits to the clinic or hospital. Radioactive 

 elements prepared by the Berkeley cyclotron are distributed (usually 

 by airplane) to various parts of America for use in hospitals and 

 laboratories. In 1942 there were 13 cyclotrons in the United States 

 and 9 planned or in building. There were 10 in other countries, 

 including 3 in England. 



The neutron beam provided by the cyclotron is thousands of times 

 more powerful than any other method at present can furnish. Neu- 

 trons ionize in a different manner from that of gamma- or X-rays, 

 and, since ionization and biological action are linked, the biological 

 action of neutrons on normal and diseased tissues constitutes a new 

 and wide field of research. 



These three uses of the cyclotron in biological research will now be 

 considered in greater detail. 



TAGGED ATOMS IN TRACER SUBSTANCES 



Radioactive or tagged atoms are used to enable the investigator 

 to see where the rest of his material is going — in just the same man- 

 ner as tracer bullets are used in antiaircraft or other gunfire (16). 



The method was used first by Hevesy (17) in 1923 when he em- 

 ployed radium D as a tracer for its isotope lead, and later radium 

 E for bismuth in his studies on plant metabolism. These substances 

 are not, however, those normally used by plants, and were admittedly 

 foreign substances introduced into the plant's economy for experi- 

 mental purposes. The great advantage of the artificially produced 

 radioactivity is that substances can be selected which are natural to 

 the plant's metabolism. One radioactive atom in a million is sufficient 

 for the progress of the substance to be followed with extreme ac- 

 curacy by physical detectors and at this dilution no biological effects 

 of the radioactivity can be detected. By "tagging" atoms of a sub- 

 stance natural to the plant or animal under observation it is possible 

 to discriminate between the added compound and similar compounds 

 already present in the system. This is a conspicuous advantage over 

 ordinary methods of chemical and biochemical analysis where added 

 substances are indistinguishable from those already present. 



It is possible to make radioactive isotopes of all the stable ele- 

 ments, and these isotopes behave identically with the commonly ob- 

 tained element both chemically and physiologically. The two differ 

 only in the fact that the artificial product is physically detectable 

 by its radioactivity in whatever chemical reactions it takes part. A 

 list of some of the more commonly used radioactive elements is given 

 in table 2 (18). 



676212 — 46 10 



