PHYSIOLOGY OF RADIATION INJURY 997 



MISCELLANEOUS TISSUES AND ORGANS 



The relative radio-sensitivities of various tissues and detailed histologic 

 information on radiation-induced changes within them are discussed by 

 Bloom and Bloom in Chap. 17. In the ensuing discussion, we shall men- 

 tion briefly certain physiologically important effects of ionizing radiation 

 on some of these tissues and organs. 



Skin. Radiosensitivity of the skin constitutes an important factor in 

 radiation therapy, mainly owing to the fact that it often imposes the 

 chief limitation on deep therapy. Since erythema is the first visible sign 

 of radiation effects on skin and runs parallel to later and more serious 

 effects, much attention has been given to it in clinical practice. Its 

 vascular basis and usual course have been discussed already (p. 976). 

 Large dosages result after a few weeks in denudation of the epithelium 

 (moist epidermititis) (Regaud and Nogier, 1913). Treatment of the 

 entire surface of the bodies of mice by /3 rays from P 32 -containing plaques 

 reproduces this state over large areas where the dosage is calculated as 

 4000 rep, beginning in four weeks and continuing for some months there- 

 after. New epidermis is continually formed under the sloughs through- 

 out this period (Raper and Barnes, 1951b). These animals show a gradu- 

 ally increasing mortality during the years after exposure. With higher 

 dosages, survival becomes much shorter, and above 5000 rep most of the 

 deaths occur within three weeks. 



It has been found in animals of different size (baby rats, mice, adult 

 rats, guinea pigs, and rabbits) that the lethal dose, expressed as the 

 45-day LD 5 o, increases with size from 2200 rep to 17,500 rep. These 

 dosages correspond to equivalent proportional volume dosages (gram-rep 

 per gram) for the several species, which are fairly comparable with the 

 respective lethal volume dosages of 7 rays (Raper, Zirkle, and Barnes, 

 1951). The reason for this correspondence is not clear; it is obvious that 

 external irradiation and penetrating ionizing radiations act through 

 different mechanisms, since they are incompletely additive (Raper and 

 Barnes, 1951c) and since lethal external /? irradiation fails to produce 

 leukopenia (Raper and Barnes, 1951a). It seems probable that certain 

 of the mechanisms of death are the same as those following thermal burns, 

 although the changes in skin responsible for them are slower in developing. 



Hair. Temporary epilation is produced in man by dosages of radiation 

 of the order of 375 to 500 roentgens (Pendergrass and Mahoney, 1948). 

 The basis for epilation is presumably inhibition of growth in the hair 

 follicles. Permanent epilation requires a considerably higher dosage. 

 Graying of hair is seen in mice following relatively low doses and is nearly 

 complete in certain areas at 1000 r (Chase, 1949). Although temporary 

 epilation is widely produced in the course of therapeutic irradiation, 

 graying is seen rarely after regrowth takes place, and it is therefore clear 

 that the human threshold for graying is considerably higher than that for 



