PHYSICAL AND BIOLOGICAL FACTORS 927 



BIOLOGICAL FACTORS IN RADIOSENSITIVITY 



Many of our concepts concerning the nature of radiation action are 

 derived from studies of radiosensitivity and of the factors that influence 

 it. The selective action of radiation on different parts of the same cell 

 and on different cells and the relation between differentiation, mitotic 

 activity, and radiosensitivity were described during the first decade 

 after Rontgen's discovery of X rays. There followed numerous attempts 

 to modify sensitivity experimentally. These included the effects of 

 temperature, oxygen, blood flow, and hydration (Schwarz, 1909; Hol- 

 thusen, 1921; Petry, 1922; Mottram, 1924; Jolly, 1924). Temperature 

 studies were especially prominent, since temperature was a convenient 

 tool, and it appeared that the radiation response was dependent upon 

 metabolic activity. Unfortunately, there were few attempts to dis- 

 tinguish the events occurring during irradiation from those taking place 

 after the exposure. Moreover, the early studies were restricted largely 

 to isolated cells and tissues. These considerations, along with increasing 

 knowledge of radiochemical reactions, have led, in recent years, to a 

 renewed interest in the factors of radiosensitivity. 



CELL AND TISSUE SENSITIVITY 



The inherent difference in sensitivity of various cells and tissues 

 attracted early attention, and in 1906 Bergonie and Tribondeau formu- 

 lated the principle that actively proliferating tissues are the most sensitive 

 to radiation and that the radiosensitivity of a tissue varies inversely with 

 the degree of differentiation. While this view has been generally 

 accepted, it is true only in a broad sense for there are many notable 

 exceptions. In contrast to the large body of information pertaining to 

 cell and tissue sensitivity is the paucity of early investigations concerned 

 with the comparative lethal dose for animals of the same and different 

 species and with the factors that influence the lethal dose. Our knowl- 

 edge relating to this aspect of the problem is, therefore, largely a 

 product of studies that have been conducted during the past 10 to 

 15 years. 



Before discussing the radiosensitivities of particular cells and tissues, a 

 discussion of what is meant by radiosensitivity is in order. If atrophy of 

 tissues is referred to, it is obvious that this may be merely a consequence 

 of inhibition of growth of tissues, which are continually regenerating to 

 compensate for cells thrown off. This is particularly true of blood-form- 

 ing organs. Interference with other synthetic processes (e.g., secretory 

 activities) may be less obvious on cursory examination. In general, 

 radiosensitivity of cells, as discussed here, will refer to destruction or 

 degeneration of cells as living entities, and it may be well to recall that 

 these events are prone to occur at the time of cell division. 



