Sec. 29.6] ISOTOPES IN THERAPY AND DIAGNOSIS 541 



also possible applications in the interstitial infiltration of malignant growths, 

 for which chromic phosphate [P10], colloidal zinc [Zn2-4], and colloidal 

 gold [Au2— 1] have already been employed in preliminary work with promising 

 results. 



29.6. Other Isotopes. Although phosphorus, iodine, and sodium radio- 

 isotopes are the only artificial species for which therapeutic applications have 

 been definitely developed, it seems likely that in a few years a number of 

 different isotopes will have well-accepted clinical uses. 



Radiostrontium has been suggested and in a few cases tried [Srl4] in the 

 therapy of bone malignancy because the behavior of this element is analogous 

 to that of calcium, which of course concentrates in bone. Although a com- 

 prehensive report is lacking, it seems unlikely that diffuse irradiation of bone 

 and marrow for relatively localized malignant growths will prove of much 

 value, particularly in view of the danger of marrow depression. 



Recently Co 60 has been suggested as a substitute for radium. The fact 

 that it can be used as the free metal, can be worked into the desired shape, 

 and has a usefully long half-life makes this use an attractive possibility. 



In addition to the elements mentioned there are important possibilities in 

 the labeling of organic compounds with long-lived radiocarbon C 14 and with 

 tritium H 3 . If artificial radioactivity is ever to prove a vital tool in the cure 

 of malignancy, the only way for accomplishing this end would seem, from our 

 present perspective, to be the discovery of organic substances that undergo 

 highly selective localization in various types of cancer cells in contrast to 

 those of normal tissue and that can be labeled with a suitable radioisotope. 

 The use of C 14 must be approached with caution because of the possibility 

 of its long-term deposition as carbonate in bone [C25] and the consequent 

 danger of induced malignant degeneration. Tritium with the low penetrat- 

 ing power of its beta particles and its rapid turnover in the body is particu- 

 larly promising because if it can be put in relatively stable positions in 

 localizing compounds, it will expend most of its ionizing power within the 

 concentrating cells. Potentially it is an ideal radiotherapeutic agent. 



The therapeutic use of fissionable isotopes, such as U 235 and Pu 239 , and 

 other species capable of releasing large amounts of ionizing radiation upon 

 being bombarded with slow neutrons, such as Li 6 and B 10 , does not seem very 

 promising because, even though it may prove possible to localize them at 

 specific points in the body, the short mean path of thermal neutrons, the 

 damaging effects of their infiltration, and the toxic effects of fissionable mate- 

 rials and their products all pose serious, if not insuperable, difficulties. 



Finally there should be mentioned the possible therapeutic use of charged 

 nuclear particles — protons, deuterons, and alpha particles — to high energies 

 in the cyclotron and other heavy-particle accelerators. This would be an 

 isotopic application of sorts inasmuch as the nuclei of single species of hydro- 



