538 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 29 



in rapidly growing tissue (particularly in the nucleoprotein) and in bone; 

 iodine localizes in the thyroid gland (at least several hundred times the con- 

 centration in the rest of the body); and certain colloidal preparations can 

 be made that localize in various parts of the reticuloendothelium. 



So far, all techniques of radiation with isotopes, except in the case of the 

 treatment of hyperthyroidism with radioiodine, have suffered from the 

 limited selectivity of uptake by the diseased tissue over normal tissues. For 

 example,- in the treatment of diseases of the bone marrow and lymphatic 

 system with radioactive phosphorus it is impossible to avoid to a greater or 

 lesser extent the irradiation in various parts of the body of those normal cells 

 that metabolize rapidly and others in the immediate vicinity of the latter. 

 Actually in the treatment of polycythemia vera it would be desirable if irradi- 

 ation could be delivered only to the nucleated red cells and in lymphosarcoma 

 and leukemia only to the offending leukocytes or lymphosarcoma cells, and 

 not to the normal cells, especially the platelet and red-cell-producing centers. 

 Some progress may be anticipated in the management of these blood dys- 

 crasias by the application of labeled colloids. Recent work [P228] shows that 

 radiation may be limited largely to the liver and spleen by the use of anhy- 

 drous colloidal chromic phosphate labeled with P 32 , which localizes selectively 

 in the reticuloendothelial cells of the liver and spleen when introduced intra- 

 vascularly in a highly dispersed form. Similar properties have been dis- 

 covered [Zr3] for a number of other colloids — of yttrium, zirconium, and 

 columbium — which may be labeled with radioisotopes of these respective 

 elements. Some of the colloids can be concentrated to a high degree in the 

 bone marrow, others in the liver and spleen. 



However, for neoplastic diseases in general, localizing substances of ade- 

 quate selectivity that can be labeled and used for radiotherapy have not been 

 found. 



The availability of radioactive isotopes for internal therapy has been 

 critically evaluated recently by Hahn and Sheppard [Gen66a], who established 

 the following criteria of therapeutic usefulness: 



1. An isotope should be pure and free from contaminants. 



2. The "half-life" should be less than 10 days and more than 2 days. 



3. The physical and chemical properties must be known and the biological 

 behavior understood. 



4. Localization and selectivity in deposition are necessary. 



Only 18 isotopes could be listed from the 450 known in 1944 (Seaborg's 

 table). There are, however, other isotopes {i.e., Co 60 ), which may be useful 

 by being removable after therapeutic application, just as is radium. 



The above criteria must be met in part at least for isotopes to be used in 

 diagnosis, but, because tracer rather than therapeutic amounts would in 

 most cases be used, the potential limitations are scarcely comparable. 



