CARCINOGENESIS BY IONIZING RADIATIONS 1175 



transplantable leukemic tumor which tends to remain localized in normal 

 hosts at the site of injection will disseminate in irradiated hosts, causing 

 generalized leukemia. 



SKIN 



This organ is more resistant to induction of neoplasia by irradiation 

 than most internal organs. Single or repeated exposures to X or 7 radia- 

 tion which are harmless to the skin may cause neoplasms of internal 

 organs; leukemia in man; leukemia, ovarian, mammary, and lung tumors 

 in mice; mammary tumors in guinea pigs; and uterine tumors in rabbits. 

 Nevertheless cutaneous tumors were the first noted in man and repro- 

 duced in animals, mainly because of the relatively soft type of X radiation 

 used earlier and due to lack of adequate filtration. While penetrating 

 radiations, irrespective of type, produce internal tumors and leukemias, 

 the carcinogenic effect of nonpenetrating radiations is limited to the 

 skin because this is the site of greatest absorption (see also Blum, Vol. 

 II, this series, in preparation). 



During the first decades following the discovery of X radiation, radia- 

 tion cancer of the skin occurred among those professionally exposed, as 

 has been described, but in recent years therapeutic exposure has been the 

 more common cause (Saunders and Montgomery, 1938). Self-produced 

 radiodermatitis has not vanished, however. In one large hospital alone, 

 115 physicians were treated for radiodermatitis, and 39 for skin cancers. 

 Carelessness and ignorance are the two main causes of these preventable 

 conditions (Leddy and Rigos, 1941). Skin tumors do not develop follow- 

 ing irradiation without an attendant reversible inflammation. Chronic 

 radiodermatitis is encountered particularly as a result of the injudicious 

 irradiation of various benign dermatoses. The principle that the more 

 extensive the injury, the more likely the superimposition of cancer, is well 

 established. Epitheliomas apparently develop with equal frequency 

 from either keratoses or ulcerations (Saunders and Montgomery, 1938). 



In man the neoplasms arising in the epidermis usually commence with 

 a warty growth (papilloma) and terminate in squamous-cell carcinoma. 

 Ulceration usually but not invariably precedes the development of 

 ■carcinoma. Rarely, tumors (sarcomas) arise in the connective tissue. 

 The first experimental tumors induced in rats were sarcomas, in the 

 rabbit carcinomas, and this may be due to differences in the texture and 

 thickness of the skin. The studies of Wolbach (1909) on the early histo- 

 logical changes in skin leading to carcinoma are now classical. 



The earliest changes recognized by Wolbach were in the collagen. The 

 most conspicuous and constant change in connective tissue is rarefaction 

 immediately beneath the epidermis and a greater density in deeper layers 

 (Saunders and Montgomery, 1938). There is homogenization of the 

 collagen with formation of dense sclerotic areas taking on a bluish color 

 with hematoxylin and eosin stain. Wolbach found degenerative changes 



