CHAPTER 16 

 INTERNAL DOSIMETRY 



16.1. Physical Principles of Dosimetry. The essential purpose of dosim- 

 etry is the quantitative evaluation of some effect produced in tissue by a 

 given quantity and type of radiation. The radiations with which internal 

 dosimetry is chiefly concerned are those emitted by radioactive materials 

 distributed in or near tissue. The principles and methods outlined in the 

 following sections, however, apply equally well to other kinds and sources of 

 radiation including x-rays and neutrons, protons, deuterons, and alpha 

 particles obtained from high-energy accelerators. 



The first requirement of dosimetry is an index on which a unit of dose can 

 be established. In the most general terms the index may be a biological 

 indicator, some physical or chemical effect produced by the radiation, or 

 some property of the radiation itself. The ultimate information desired is, 

 in nearly all cases, the biological or clinical effects produced by a given 

 radiation or source of radiation. As yet, however, no biological indicator 

 has proved satisfactory as a unit of dose. Factors such as mean lethal dose, 

 chromosomal changes, and mitotic activity may be taken as biological indi- 

 cators but are useless as units of dose. They do not lend themselves to 

 convenient measurement and not at all to calculation. Different tissues 

 and organs as well as different species of animals vary widely in their resist- 

 ance to radiation so far as observable biological or clinical results manifest 

 themselves. Moreover, for any one tissue exposed to a given energy flux 

 of radiation (mev or ergs per square centimeter per second), observable 

 biological effects are strongly dependent on both the type and the energy of 

 the radiation. For these reasons an index based on any one biological effect 

 would in practice be difficult to correlate with other effects and with similar 

 processes in different tissues. This applies equally well to chemical effects 

 induced by radiation since they are subject to the same shortcomings. 

 There remain, consequently, only physical properties of either the radiation 

 itself or its interaction with tissue. 



From a physical point of view the ideal unit of dose is either the energy 

 absorbed from the radiation per unit mass of tissue or the ionization formed 

 per unit mass. Both these quantities can be measured with reasonable 

 accuracy and may be expressed in absolute units. Since units of energy 



403 



