536 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 28 



contribute to the radiotoxicity of the parent elements). No attempt has 

 been made here to cover the extensive literature on the toxicology of thorium 

 and uranium, nor on the application of thorium (as thorotrast) in radiography. 

 Studies with the other five actinides have been recent and as yet rather 

 limited. However, a great deal of work may be expected on plutonium, 

 an element of which the isotope Pu 239 is made artificially in large quantities 

 in the uranium-pile reactor but which exists in nature only in minute traces. 



Hamilton's comprehensive study of the metabolism of fission elements was 

 done simultaneously with study of the six actinides — thorium, protacti- 

 nium, neptunium, plutonium, americium, and curium [Pal, etc.]. He has 

 found that like lanthanum, germanium, praseodymium, and prometheum 

 they concentrate to a high degree in bone when administered parenterally, 

 but are not absorbed in significant amounts from the digestive tract. Radio- 

 autographic study has demonstrated that bone deposition does not occur 

 uniformly in the osseous tissue (as with strontium), but rather in the per- 

 iosteum and endosteum, and in the region of the trabecular bone. Recent 

 work has also shown that Pu 239 (24,000-year half-life) in bone can produce 

 malignant change [PulO]. 



28.7. Other Elements. Elements not already covered in this chapter 

 include two light elements, scandium and titanium, for which no tracer work 

 has been done, and a number of the heavier elements above the lanthanide 

 series, — namely, hafnium, tantalum, rhenium, osmium, iridium, platinum, 

 thallium, polonium, astatine, and francium. Of the latter only polonium 

 and astatine have been studied biologically. 



Only two studies [Pol, 2] on the distribution and biological effect of polo- 

 nium have been carried out with the isotope Po 210 , a member of the uranium 

 series. The element concentrates in the reticuloendothelial system, especially 

 of the spleen. The first autoradiographs were taken with the use of tissue 

 slices containing polonium [Pol]. 



Astatine has been studied with the isotope At 211 (7.5-hr half-life). In 

 the one published paper dealing with this species [Atl] it has been found to 

 resemble its analogue iodine quite closely in being concentrated to a high 

 degree in the thyroid. 



