538 L. H. GRAY 



metrically when the dose is large and the duration of irradiation short, 

 and even in such cases it would be unwise to attempt to measure the 

 rise in temperature of the biological specimen itself since this might be 

 seriously disturbed by an alteration in cellular metabolism initiated 

 by the radiation. The rise in temperature of an object representing 

 the biological specimen in size and composition might be measured, 

 but here again care must be exercised with regard to thermochemical 

 change. 



1. Units of Dose 



The Roentgen. Since it is impossible to measure small doses 

 calorimetrically alternatives have to be sought. The problem first 

 became urgent more than 25 years ago in connection with the meas- 

 urement of X-ray dose in radiotherapy and the most practicable solu- 

 tion was found to be to attempt to measure the ionization produced 

 by the X radiation in air under conditions that made it possible to infer 

 the amount of X-ray energy that would be absorbed per unit mass of 

 air in the position occupied by the portion of tissue under considera- 

 tion. The quantity measured is therefore the ionization produced in 

 air under standard conditions, and the unit of X-ray dose, the roent- 

 gen, is defined as "that quantity of X or 7 radiation such that the 

 associated corpuscular emission per 0.001293 gm. of air produces, in 

 air, ions carrying 1 electrostatic unit of quantity of electricity of either 

 sign." 



The usefulness of the roentgen as a unit of X- or 7-ray dose de- 

 pends on three things. First, that the total number of pairs of ions 

 formed by X rays per unit mass of air bears an approximately constant 

 ratio to the total X-ray energy absorbed per unit mass of air over prac- 

 tically the whole range of X and 7 radiation. Secondly, that at least 

 to a first approximation the absorption of X and 7 rays per unit mass 

 of air is equal to the absorption per unit mass of tissue, and even a 

 rough knowledge of the quahty of the X radiation and the composition 

 of the tissue enables the ratio of the two to be estimated with accu- 

 racy. Thirdly, the range of the secondary electrons that actually 

 ionize the tissue is only a few millimeters even when generated by 7 

 radiation so that the ionization is produced in tissue at a point not 

 far removed from that at which X- or 7-i'ay energy is absorbed. The 

 pattern of actual energy absorption throughout the bulk of tissue is 

 thus not greatly different from the pattern of the distribution of dose 

 measured in roentgens. 



