120 RADIATION BIOLOGY 



and studies of kinetics provide much of the available evidence. Even 

 the understanding of many macroscopic physical effects, such as the 

 induced emission of light by phosphorescent materials, is still rather 

 tentative. 



As a preliminary approach we consider here briefly how the magnitude 

 of radiation treatments and of their macroscopic effects are expressed. 



5-la. Radiation Dose. The main quantitative characteristic of a 

 radiation treatment is, of course, the amount of radiation received by the 

 treated material. This amount is indicated loosely as the "dose." The 

 concept of dose relates to the amount of action actually produced within 

 a material rather than to the mere passage of radiation through the 

 material. 



It would be desirable to express the dose in such a way that the simple 

 indication of the dose of a treatment gives an adecjuate indication of its 

 biological effectiveness, without any need for a detailed specification of 

 the kind of radiation employed. This goal can be achieved at least for a 

 broad range of X-ray qualities (see Sect. 5-5) but certainly not for all 

 ionizing radiations. Failing this possibility, it appears convenient to 

 express the dose by some quantitative index of the initial physical action 

 of radiation on a material, which may serve as an unequivocal standard 

 of reference. The 6th International Congress of Radiology recommended 

 in 1950 that the dose should be expressed, in principle, in terms of energy 

 dissipation: "For the correlation of the dose of an ionizing radiation with 

 its biological or related effects the International Commission on Radio- 

 logical Units recommends that the dose be expressed in terms of the 

 quantity of energy absorbed per unit mass (ergs per gram) of irradiated 

 material at the place of interest." 



This recommendation does not exclude other methods of expressing 

 the dose, but care should be taken to avoid misunderstanding. 



For example, the 1950 recommendation does not supersede the pre- 

 viously established practice of expressing doses in terms of the ionization 

 produced in a standard material, namely, air. This practice is quite 

 normal for X rays of low and moderate energies (up to a few million 

 electron volts), whose dose is currently expressed in roentgen units. 



The definition of the roentgen accepted by the 5th International 

 Congress of Radiology in 1937 relies on the following principle: The 

 energy of all electrons released in a certain mass of air, by photoelectric 

 effect, Compton scattering, or pair production, serves as an index of the 

 dose of X or 7 rays. This quantity of energy is characterized, in turn, by 

 the amount of ionization which the electrons produce. "The roentgen 

 shall be the quantity of X or 7 radiation such that the associated cor- 

 puscular emission per 0.001293 g of air produces, in air, ions carrying 1 

 electrostatic unit of cjuantity of either sign." 



The use of the roentgen will probably persist in the field of X and 7 



