EFFECTS OF WHOLE-BODY IRRADIATION 



257 



Figure 9-10. The "Theratron Junior," Typical of Co 60 "Bombs" Used in Cancer Clinics in 

 Many Countries. Source is contained in the lead head (above), and radiation is collimated 

 by a tubular hole. Lead absorber and counterweight is below. Both source and patient 

 can be moved so that the patient can be irradiated from several directions. Typical charge 

 is 1000 curies of Co 60 , which gives about 15 roentgens per minute at a spot 1 meter from 

 the source (15 rmm). (Photograph courtesy of Atomic Energy of Canada Ltd.) 



On the other hand, some atoms such as I 131 will fit nicely into the biochem- 

 istry of the body, localize in the thyroid, and irradiate it with betas and 

 gammas (refer to Table 5-7). 



However, there is a basic difference between the two methods of applica- 

 tion of irradiation. Machine-made X rays, or the gamma rays from a 

 cobalt-60 bomb provide either a constant dose rate or one which can be 

 varied at will by the radiologist. By contrast, radioactive isotopic therapy 

 depends upon the biochemistry of the system to transport the injected iso- 

 tope to the locale to be irradiated, and then to excrete it. If the application 

 of the isotope is direct (see Figure 5-9), or if the induction time is short, the 

 isotope has a biologically effective half-life, / cff , which is the half-time of ir- 

 radiation. In any case, the dose rate, dD/dt rads per sec, is proportional to 



