1172 RADIATION BIOLOGY 



of thyroid-stimulating hormone circulating in the blood. Some of these 

 thyroid remnants exhibit the morphological appearance of a precancerous 

 lesion. Doniach (1950) administered 32 /xc of I 131 either alone or in 

 combination with methylthiouracil to rats, and thereby increased the 

 rate of formation of thyroid adenomas and, in one rat, a thyroid tumor 

 was observed. Similarly, Goldberg and Chaikoff (1951a) observed multi- 

 ple adenomas in two of ten rats that had been given a single injection of 

 400 juc of radioactive iodine. 



In a later work, Goldberg and Chaikoff (1952) observed malignant 

 thyroid tumors in seven of twenty-five rats given a single injection of 

 400 juc of radioactive iodine. Metastases occurred in all seven rats, the 

 organs invaded being lung, adrenal, lymph nodes, skin, and bone. 



Invasion of blood vessels, and even lung metastases, were seen by 

 Morris and Green (1951) following long-continued administration of 

 thiouracil to mice. Many of these tumors ultimately became autono- 

 mous in the sense that they grew in normal mice (Morris et al., 1951). 



The very definition of malignancy is now controversial. It is no longer 

 possible to determine on the basis of gross or microscopic studies whether 

 such lesions, so readily induced in different endocrine organs by irradia- 

 tion, possess autonomy, or are nonneoplastic growths caused and main- 

 tained by derangement of the normal hormonal balance. Further 

 studies on the induction of neoplasia of the thyroid gland by I 131 alone 

 or in combination with goitrogens are desirable. On the basis of related 

 experience, one might expect induction of thyroid tumors in a high per- 

 centage of the experimental animals by a dose below that which com- 

 pletely destroys this organ. The origin of autonomy in conditioned 

 growth has been demonstrated in several organs, including the thyroid 

 and the pituitary (Furth et al., 1952). 



In man, Kindler (1943) described the development of thyroid carcinoma 

 and that of the hypopharynx following massive prolonged irradiation of 

 the neck. After discussing the literature, he concludes that "Uberdosier- 

 ung erzeugt mit Sichercheit Krebs." 



Parathyroid. This appears to be resistant to radiation. No tumor has 

 ever been described in this organ, either after total-body or local irradia- 

 tion. When mice are radiothyroidectomized with I 131 , this organ must 

 also receive large quantities of ionizing radiations. It usually decreases 

 in size and often little of it is found in serial sections; yet neither neo- 

 plastic nor preneoplastic changes have been noted in this organ (Furth, 

 unpublished data). 



MAMMARY GLAND 



There are numerous reports on the induction of mammary tumors by 

 ionizing irradiations in several species of animals (mice, guinea pigs, and 

 rats). The induction of this tumor appears to take place by an indirect 



