RADIOACTIVE TRACERS 865 



which in turn is influenced by the amount of available oxygen (Laser, 

 1954). Gray, Conger, Ebert, IIorney and Scott (1953) that observed 

 the sensitivity of the ascites tumour cells of the mouse to X-rays is 

 about three times as great when irradiatc^dina well-oxygenated medium 

 as it is under anoxic conditions. This observation may prove to ))e of 

 great importance to the radiotherapist. 



HAEMOPOIETIC ARREST 



Anaemia is the classic radiation disease. In the iieaithy man daily 

 0.85 per cent of the circulating red corpuscles complete their life-cycle 

 and have to be replaced. Radiation interferes with the formation of reel 

 corpuscles and may accelerate their disappearance from the circulation 

 as well. It takes clays before the anaemia can be ascertained by counting 

 of the red corpuscles, determination of the corpuscle volume or the 

 haemoglobin content of the blood. By following the effect of incorpora- 

 tion of radioiron into haemoglobin of the circulation or that present in 

 the bone marrow% the haemopoietic arrest produced can be ascertained 

 at an earlier date, in the rat by investigating the ^^Fe content of the 

 circulating haemoglobin, after six hours. Only a few minutes after 

 intravenous injection of ^^Fe a slight amount of radioiron can be detected 

 in the haemoglobin of the circulation amounting in man to 0.3 per cent 

 after the lapse of one hour (Dal Santo, 1956). This activity, however, 

 is presumably due to the incorporation of-^^Fe into the haemoglobin of 

 the circulating reticulocytes. 



Hennessy and Huff (1950) were the first to show that the rate of 

 appearance of administered radioactive iron (^^Fe) in the circulating red 

 corpuscles was considerably depressed by even small doses of whole 

 body radiation given 24 or 48 hours before the injection of iron. This 

 result was repeatedly confirmed. As can be seen in Fig. 9, which is 

 taken from a paper by Baxter, Belcher, Harriss and Lamerton 

 (1955), the exposure of rats to a dose of 24 r already has a marked effect 

 on the incorporation of subcutaneously injected ^^Fe in the haemoglobin 

 content of the circulating blood, and thus on haemoglobin formation. 



The depressed rate of formation of haemoglobin could be due to an 

 interference with the biochemical synthesis of haemoglobin or of haemin 

 in the erythropoietic marrow cells, or alternatively to a mitotic inter- 

 ference or a lesion of the latter. If haemin synthesis per se would be 

 radiosensitive we would expect the formation of other haemins present 

 in the organism, that of myoglobins, catalases, cytochromes, to be 

 depressed as well after exposure of the animal to radiation. This line of 

 thought induced us to follow the effect of irradiation on the incorporation 

 of ^^Fe, not only into haemoglobin, but also into the above mentioned 



0'5 Hevesv 



