Dosage determination in the use of radioactive isotopes.

As commonly used, most radioactive isotopes are ingested or injected in soluble form and subsequently deposited with a greater or lesser degree of selectivity in various cells and organs. In the event of a single administration, the number of radioactive atoms present at any particular site will increase at first (as they are drawn from the circulation along with their stable isotopes), it will reach a maximum and, as the source of supply becomes exhausted, it will decrease on account of both metabolic turnover and radioactive decay. The latter is accompanied by the release of ionizing radiation, the type and emission rate of which is the exclusive characteristic of the radioelement itself. The direct measurement of radiation released in this manner in terms of units already established in Radiology is beset with numerous difficulties of experimental nature. Nevertheless when the physical factors of half life and radiation energy, and the physiological factors of uptake and excretion, are known, it is possible, in some cases at least, to make satisfactory estimates of tissue dosage. The roentgen, as defined by international agreement, applies only to xor gamma radiation; it can therefore be used for gamma ray emitting isotopes but not for radiation due to primary beta particles. (The roentgen is defined as "that quantity of xor gamma radiation such that the associated corpuscular emission per 0.001293 gram of air produces, in air, ions carrying one e.s.u. of quantity of electricity of either sign.") On the other hand if the energy absorbed per gram of air per roentgen (a--. 83 ergs) is made the unit of energy absorption for beta rays, it is possible to establish a comparable basis for beta ray dosage. To be sure, in going from air to tissues certain corrections will have to be made because the energy absorption in tissue per roentgen exposure of xand gamma rays depends