CARCINOGENESIS BY IONIZING RADIATIONS 1191 



The neoplastic change is explained as an anomaly of cell division. Lacas- 

 sagne distinguishes between equipotential mitosis, from which arise 

 similar daughter cells, and differentiated mitosis, which yields dissimilar 

 daughter cells. The malignant transformation is related to alteration of 

 the reproductive apparatus of the cells, fixing their descendants and 

 causing preponderance of equipotential mitosis. 



The tumors arising in the endocrine organs (e.g., pituitary) represent 

 another extreme type of growth in which little if any change in character 

 of the cell has to be postulated, since multiplication of these tumor cells, 

 like that of their normal homologues, is caused by a sustained endocrine 

 stimulus. Since they metastasize, they behave as malignant growths, 

 yet they may be checked by correction of the hormonal disturbance 

 (Furth et at., unpublished data). It may be disputed that such condi- 

 tioned growths could be classified as tumors. On the other nand, 

 ignorance of the processes on which the origin and sustained growth of 

 common neoplasms may depend is no guarantee that such processes do 

 not exist. In any event the known dependent growths gradually or 

 suddenly lose the dependency and sooner or later become autonomous. 



The statement that irradiation may hasten the aging process and thus 

 bring about an earlier appearance of spontaneously occurring tumors does 

 not seem to hold for all neoplasms. Leukemia can be induced in young 

 animals by irradiations and other carcinogens with high frequency in 

 strains of mice in which this disease is exceedingly rare (e.g., in strain 

 C57 black). Many neoplasms occur in young people and are of types 

 not caused by ionizing irradiations. On the other hand, many tumors 

 observed induced by irradiations are of the same type as found 

 spontaneously. 



The induction of neoplasms by ionizing irradiation is a fascinating 

 chapter in the history of science. It is full of problems of increasing 

 practical importance awaiting solution. As a tool in cancer research, it 

 is unequaled. 



REFERENCES 



Most of the older literature, well reviewed in the monographs of Lacassagne 

 (1945a, b) and Colwell and Russ (1934), is omitted for reasons of economy. Of the 

 clinical reports, only those of historical interest and the more recent ones are listed, 

 notably those which cite the older literature. Of the publications of authors with 

 numerous articles, only the first, the last, and the more comprehensive ones are listed. 



(Information on availability of government reports indicated by an asterisk may 

 be obtained from the Office of Technical Services, Department of Commerce, 



Washington, D.C.) 



Abrahamson, L., M. H. O'Connor, and M. L. Abrahamson (1950) Bilateral alveolar 

 lung carcinoma, associated with the injection of thorotrast. Irish J. Med. Sci., 

 6: 229-235. 



