474 BIOLOGICAL EFFECTS OF RADIATION 



in dosage methods used by the different investigators. Until the inter- 

 national unit of measurement of roentgen radiation, the roentgen (r unit), 

 was agreed upon, these dosage factors could only be identified in a 

 general way. Since the adoption of this r unit, it is possible to compare 

 dosages applied to the surface, at least, with one another. The roentgen 

 postulates that ionization effects of roentgen radiation are comparable, 

 but so far methods of measuring such effects over the other parts of the 

 spectrum have not been available. The situation is still obscured if the 

 unit erg/cm. 2/sec. is used; this defines the total quantity delivered upon 

 a certain plane or surface but does not allow for that lost by transmission 

 through the tissue. Nor does it take into consideration any specific 

 properties of the substance irradiated, i.e., specific spectral absorption 

 or transmission in the ultra-violet spectrum. Perhaps the matter would 

 be simplified still further if some common energy equivalent like ergs 

 absorbed per unit volume of tissue were used, which could be applied 

 with greater accuracy to the whole spectrum. 



Another difficulty encountered in comparing the physiological effects 

 of these radiations is the variation in the intensities and the time factors 

 involved, and the lack of a clear understanding of the differential effects 

 of the different bands of the electromagnetic spectrum from the shortest 

 to the longest wave-lengths. Most of these differences may be based 

 upon purely physical rather than biological phenomena. In addition, 

 it is often difficult to interpret the results obtained, because of the failure 

 of many of the experimenters to take into consideration the broader 

 concepts of the effects of radiation, the authors having emphasized the 

 clinical and empirical rather than the biological and physical viewpoints. 

 Further, there is still a lack of agreement in the establishment of a definite 

 biological unit of dosage, chiefly because there may be certain individual 

 differences, as well as species differences, in the test objects, which may 

 influence the effectiveness of a given dosage. Moreover, results obtained 

 with experimental animals may not be directly applied to human tissues, 

 although in general the principles are the same, however different the 

 doses involved may be. 



The primary effect of radiation may be considered to be a destructive 

 one. Secondary effects, aside from loss of function, are mainly those 

 due to repair of this damage. The whole concept of radiation therapy 

 is built upon the supposed greater sensitivity of the tumor cells or the 

 abnormal cell mass treated over that of the normal near-by structures, 

 of necessity exposed to the radiation. Normal tissues, too, vary greatly 

 in their susceptibility to damage by radiation, and only rough estimates 

 are available for a comparison of the relative doses involved in tissue 

 damage. 



The variation in the depth dosage in the tissues irradiated in experi- 

 mental animals and humans must bear some important relation to the 



