992 RADIATION BIOLOGY 



to the lethal action of X rays is especially striking with low dosages. 

 The degree of potentiation of radiation toxicity will, of course, depend 

 upon the nature of replacement therapy and the time intervening between 

 adrenalectomy and irradiation as well as the completeness of adrenal 

 extirpation. It is significant that survival of adrenalectomized mice is 

 similar to that of animals with intact adrenals when a constant dose of 

 adrenal cortical extract is given daily (Straube et al, 1949). 



While there may be some rationale for employing adrenal corticoids to 

 reduce radiation mortality, for the most part the results of such endeavors 

 have been rather disappointing. Desoxycorticosterone has been shown 

 to prevent fatty infiltration of the liver and to improve survival in mice 

 (Ellinger, 1946, 1947). There are also some reports of its efficacy in 

 clinical radiation sickness (Weichert, 1942; Ellinger et al, 1949). These 

 observations have not been verified by others; a number of preparations, 

 including desoxycorticosterone, 11-dehydrocorticosterone, whole adrenal 

 cortical extract, and cortisone have been used experimentally without 

 success (Swift et al, 1948; Straube et al, 1949; Graham, Graham, and 

 Graffeo, 1950; Graham and Graham, 1950; W. W. Smith et al, 1950). 

 This is true also of the adrenocorticotrophic hormone (W. W. Smith et al, 

 1950). It is well to recall that negative results have been obtained with 

 adrenal extracts in other conditions of stress. It remains to be deter- 

 mined whether this obtains because of the difficulty in approximating 

 the quality and quantity of the naturally occurring internal secretions. 



Thyroid. The normal thyroid is quite resistant to ionizing radiation in 

 contrast to the relative sensitivity of the hyperplastic gland. Heavy 

 irradiation, more than several thousand roentgens, is required to induce 

 structural changes in the thyroids of the guinea pig, rat, rabbit, and dog 

 (Shields Warren, 1943b; Bender, 1948). It has been found, for example, 

 that local exposure of the thyroid area of young or mature rats to 50 or 

 5000 r of X rays does not alter the histological appearance of the gland, 

 nor modify basal oxygen consumption or body weight (Bender, 1948). 

 Uptake of radioiodine is also unaffected by local irradiation with 1000 r 

 and is actually increased with 3000 and 6000 r (Hursh et al, 1949). 

 Increased uptake of I 131 with the higher dosages occurs in the absence of 

 morphological changes and is regarded as an indirect result of irradiation. 

 The level of local X irradiation necessary to produce significant destruc- 

 tion of thyroid tissue in the rat is apparently greater than the highest 

 dosage that can be administered without a fatal outcome (Bender, 1948; 

 Hursh et al, 1949). Dosages in excess of 5000 r applied to the thyroid 

 area of the rat invariably result in death within about a week. The only 

 grossly abnormal feature is dehydration, which has been attributed to 

 failure to consume food and water for several days preceding death. 

 While death may be delayed by suitable administration of fluids, the 

 mechanism responsible for the lethal action is undetermined. It is 



