258 BIOLOGICAL EFFECTS OF IONIZING RADIATIONS 



the average energy*** of the emission and to the strength of the source, c 

 microcuries. Thus 



dD/dt = 5.92 x lO- 4 ^ 



where the constant arises from the definitions of the curie (3.7 x 10 10 disin- 

 tegrations per sec) and the rad (100 ergs absorbed per gram), and the fact 

 that 1 Mev = 1.6 x 10~ 6 ergs. In the case in which / eff is shorter than the 

 physical half-life of the isotope, the dose received integrates to 



D B {t) = 74 V ; eff (l - r»«W) 

 for any time t; or 



D & {*) = 74 E c o t efr 



for the total dose administered (by an initial concentration, c microcuries, 

 of a beta emitter with an average energy E g Mevs and a biological half-life 

 oH eff days) up to the time the isotope has been practically completely ex- 

 creted. Table 9-6 gives pertinent data for different isotopes and organs. In 

 some cases ^ eff is limited by rapid chemical turnover, in others by the decay 

 half-life. Note that only a fraction of an isotope accumulates at a particular 

 locale in the system. Therapy depends upon preferential uptake by an organ. 

 The rest of the system gets irradiated too, but less. 



P 32 has been used successfully for the irradiation of excess white (leuke- 

 mia) and red (polycythaemia) blood cells. Other isotopes are being used in 

 ever-increasing numbers and amounts as new techniques (e.g., the insertion 

 of radioactive colloidal material (Au 198 , for example) into the tumor: it 

 "floats," but it cannot get into the blood stream and be washed away), and 

 as new methods of preparation and purification become known. 



The technique of bone-marrow therapy is now in an advanced state, al- 

 though its application is limited. The principle is the complete replacement 

 of irradiation-damaged marrow with that from a donor. Transplants are 

 normally limited to inbred strains or to isologous animals. However, if the 

 natural immunity reactions of human beings are completely destroyed by 

 large radiation doses first, then complete blood transplant can be successful. 

 Even so, further complications often arise later, in terms of a secondary 

 disease. Rare cases of transplant from one identical twin to another have 

 been more successful. 



An advanced technique, which may keep radiologists in business even 



***Genera!ly the average energy for gammas is about the same as the listed values, for gam- 

 mas are monoenergetic; but for betas the average energy, Eg, is approximately 1/3 the maxi- 

 mum (nominal) energy usually listed. For X rays the average energy is always well below the 

 peak value listed — about 0.3 of the nominal kvp if the soft end has not been filtered out (by, 

 say, 0.5 mm Al), and about 0.6 of the nominal kvp if it has. 



