848 THE CARCINOGENIC STIMULUS II 



ments which deposit in bone. Lung tumors have resuhed experimentally (Lisco 

 and Finkel, 1949) from tracheal intubation of radioelements. 



Unlike ultraviolet irradiation of the skin (Blum, 1950), where repeated stimula- 

 tion is necessary for carcinogenesis, a single exposure to ionizing radiations is 

 sufficient for tumor development. The latent period of tumor induction is usually 

 longer than for chemical carcinogenesis. 



That ionizing radiations may induce the development of chemical carcinogens 



OVARIAN AND ADRENOCORTICAL TUMORIGE NESIS POST-IRRADIATION 



PITUITARY GLAND 



Fig. 36. Endocrine factors 

 implicated in the induc- 

 tion of ovarian and adrenal 

 cortical tumors by X-rays. 



(2) 

 LOW BLOOD LEVEL OF 

 ESTROGENIC HORMONE 



OVARY NON-FUNCTIONAL 

 POST-IRRADIATION 



UTERINE STIMULATION 

 AFTER TUMOR DEVELOPMENT 



(3) 



HYPERSECRETION OF 



GONAOOTROPE TO INDUCE 



ADRENAL AND OVARIAN 



NEOPLASIA 



ADRENOCORTICAL AND 

 OVARIAN NEOPLASMS 



CASTRATE UTERUS 

 POST-IRRADIATION 



in the tissues is suggested by the work of Kaplan et al., (1954, 1956a, b) and 

 Carnes et al., (1956). The sequence of events in radiation-induced carcinogenesis 

 invariably involves initial destruction or hypoplasia of irradiated tissue. In this 

 regard the process is not unlike that involved in certain instances of chemically 

 induced carcinogenesis {e.g., the induction of thymomas and leukemia by es- 

 trogenic hormone). 



{b) Somatic mutation and carcinogenesis 



The induction of germinal mutations {e.g., in Drosophila) by X-rays has led 

 to speculation concerning somatic mutation and radiation-induced carcinogenesis 

 (Brues, 1954). On the basis of histocompatibility studies, mouse mammary cancer 

 cells have been considered genetically unlike homologous normal cells (Strong, 

 1929). If carcinogenic agents are mutagenic, this property would seem, however, 

 not to be specific for ionizing radiations. 



{c) Neoplasms following whole body irradiation 



After exposure of mice to total body X-irradiation, increased general tumor 

 incidence has been observed. Lung tumors occur more frequently and earlier in 

 irradiated, compared with genetically similar unirradiated mice (Lorenz, 1950). 

 Ovarian (Furth and Butterworth, 1936) and hemopoietic neoplasms (Upton and 

 Furth, 1956) are induced by whole body X-irradiation. That factors other than 

 irradiation of the target tissue are important in influencing ovarian tumorigenesis 

 and leukemogenesis is demonstrated as indicated below. 



Although irradiation of ovaries alone or of a single ovary in unilaterally ovari- 

 ectomized mice results in tumor development, the presence of functional ovaries 

 (Lick et al., 1949), in situ or grafted, or the administration of estrogenic hormone 



