1162 RADIATION BIOLOGY 



Why does a ray passing through a certain region cause the death of one kind, e.g., 

 the germinative cells of the gonads or certain cells of the lymphatic nodes, delayed 

 death of certain others, e.g., those of the epidermis, simple transient alterations 

 such as are noted in connective tissue, or no reaction at all, as in the case of nerve 

 cells. . . . Among apparently identical cells receiving equal doses of radiation, 

 some are killed, others show various lesions, still others are but slightly damaged 

 and are capable of recovery. This difference in radiation effects explains the 

 repair following irradiation, such as that of irradiated epidermis, or reversible 

 blood changes, or the reconstitution of the germinal epithelium. It explains 

 also the recurrences of cancer treated by irradiation. The traditional explanation 

 of this phenomenon is biological, viz., that the cells, in their life cycle pass through 

 stages of varying sensitivity corresponding to varying physiological conditions. 



The variable radiation sensitivity of the cell can be explained, and a means of 

 establishing this sensitivity furnished, by the discontinuous nature of radiation. 

 The sensitivity exhibited by each cell type appears to be a function of two main 

 factors: (1) the histological structure, which determines the size of the various 

 zones corresponding to each lesion, and (2) the physicochemical composition, 

 which determines the number of quanta necessary to produce ionization sufficient 

 to cause disintegration of a sufficient number of constituent molecules of the 

 intracellular organelle in question. 



These considerations apply to the induction of neoplasms at the site of 

 irradiation. Leblond et al. (1951) noted some correlation between the 

 incidence of spontaneous neoplasms of different organs and rate of repro- 

 duction of their cells. Organs with high mitotic activity appear radio- 

 sensitive, but this is not a general rule. A different mechanism is in 

 play when tumors arise at distant sites, as will be discussed. 



LEUKEMIA 



The increased incidence of leukemia among radiologists now seems to 

 be well established, and there are numerous experimental studies elucidat- 

 ing the conditions of leukemia induction in mice. 



Leukemia among Radiologists. As early as 1911 Jagie et al. described 

 four cases of leukemia among radiologists and one in a radium worker and 

 stated that long-continued exposure to X radiation may cause this 

 disease. In 1912 Aubertin stated that in nineteen years he had seen five 

 cases of leukemia among radiologists, a then relatively small fraction of 

 physicians, and none among other physicians. In 1925 Emile-Weil and 

 Lacassagne reported on myeloid leukemia caused by handling radioactive 

 substances. These reports were soon amply confirmed. Henshaw and 

 Hawkins (1944) found that during the period of 1933-1942 leukemia was 

 1.7 times more common among American white physicians than among 

 the general male white population (Dublin and Spiegelman, 1947) and 

 cited similar evidence from England and Wales. Ulrich (1946) estimated 

 that leukemia was eight times more common among American radiolo- 



