408 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 16 



volume of gas. For many purposes air is sufficiently similar to tissue to 

 use the simple ratio S — N t /N & i T ; in more accurate measurements, gas mix- 

 tures with approximately the same atomic composition as tissue can be 

 prepared. The average percentages of the principal atomic constituents by 

 weight in lean tissue, for example, are hydrogen, 9.9 per cent; carbon, 13.4 per 

 cent; nitrogen, 4.1 per cent; oxygen, 70.11 per cent; and light minerals, 

 about 1 per cent. 



Chambers based on the. cavity principle are applicable to all types of radia- 

 tion and absorbing media. They provide an accurate means for determining 

 dose in terms of absolute quantities, either ionization produced or the energy 

 absorbed expressed in fundamental physical units. When one is measuring 

 the dose in roentgens, the chamber gas and wall material, as pointed out 

 above, cannot be chosen arbitrarily but must consist of air and airlike 

 substances. 



Calculations of dose delivered to tissue by a uniformly distributed source of 

 charged corpuscular radiation (beta particles, alpha particles, and fission 

 fragments) may be expected under favorable circumstances to yield results 

 in reasonable agreement with the actual dose. Under less favorable con- 

 ditions and in most instances where x- or gamma radiation is involved, 

 calculations of dose by the methods available at present can be relied upon 

 to give only the order of magnitude of the true dose. The uncertainties 

 accompanying such calculations are due chiefly to the lack of sufficiently 

 detailed information concerning the exact disposition and concentration of 

 the radioactive isotope throughout a tissue and to the difficulty in evaluating 

 that fraction of the energy made available by radioactive decay which is 

 actually absorbed in the tissue under consideration. 



The importance of metabolism as a factor in internal dosimetry is obvious. 

 The dose delivered to specific tissues by an administered radioactive isotope 

 depends upon the amount taken up in the tissue and the length of time it 

 remains there. The metabolic paths and rates of turnover are not unique 

 for a given isotope but often depend on the chemical substance in which the 

 isotope is incorporated. These factors are known in some detail for certain 

 organic molecules that can be labeled with active isotopes, and the metabolic 

 fate of a few isotopes such as iodine, phosphorus, and sodium in inorganic salts 

 is reasonably well understood. For a few other substances it is sometimes 

 possible to determine the concentration in a specific tissue either from biopsy 

 or from previous tracer investigations. In most instances the concentration 

 does not remain constant in any one tissue, but, depending upon the com- 

 plexity of the chemical reactions the administered active substance under- 

 goes, it is regulated by the metabolic rates of uptake and elimination. From 

 the clinical standpoint, estimates of dose are further complicated by the 

 considerable variation among different individuals in the tissue mass of 



