88 Applied Biophysics 



originally suggested in 1908, and now developed into the inter- 

 national rontgen (r) of X- and gamma-ray measurement.'*" 

 Much research has been done to discover under what conditions 

 the ionization in air may be taken as a measure of the dose in 

 living tissues. '^^^ ^'^^ 



Assuming that ionization in the tissue is responsible for the 

 biological changes produced, the rontgen should be a useful unit 

 for linking physical dose with biological response, since an 

 accurate measure of any well-recognized biological response in 

 terms of the rontgen would enable the experimental conditions 

 to be repeated anywhere by any competent person. The first 

 and most obvious biological response to be recognized was the 

 erythema produced in human skin, and since the tolerance of 

 the skin to radiation is a limiting factor in many radiotherapeutic 

 procedures, the determination of the "skin erythema dose" 

 (SED) in rontgens has been the subject of much careful in- 

 vestigation.^^^' ^^^ The difficulties of such an apparently simple 

 procedure are, however, considerable. The dose received by the 

 skin is due not only to the incident radiation, but also to 

 scattered radiation which may constitute half the total dose, 

 and which varies with the quality of radiation, the size of the 

 irradiated area, and the particular part of the body (depending 

 on the relative amount of bone, muscle and fluid) being ir- 

 radiated.^^' ®'^' ^^^' ^^^' ^^^ On the biological side, the accuracy 

 of the determination is vitiated, partly owing to individual varia- 

 tion in the response to irradiation, and partly to difTerence of 

 opinion of various observers as to what constitutes the proper 

 erythema reaction. Taking the results obtained from the majority 

 of observations made, it is possible to compile tables of the 

 approximate value of the SED for different quality radiations 

 falling on a field of given area ^^^' ^^^ or volume of given size.^^^ 

 In any one series of observations made under constant conditions 

 by the same observer, the doses are likely to be comparable with 

 each other, but where different series of experiments are con- 

 sidered a comparison of doses must be made with caution. 



The determination of the SED for different quality radia- 

 tions has shown a rise in the skin tolerance as the wave length 



