530 UADIATION BIOLOGY 



generally that ultraviolet radiation is a carcinogenic agent for mice 



and rats. 



These experimental studies revived interest in the idea, already over 

 thirty years old at the time, that sunlight is a cause of cancer of human 

 skin. The earliest to suggest this were Unna (1894), Dubreuilh (1896), 

 and Sheild (1899). The most extensive observations were those of 

 Dubreuilh on the workers in the vineyards of the Bordeaux region, among 

 whom he found more cutaneous cancer than among the urban population. 

 He called particular attention to the limitation of skin cancer to the face 

 and hands, remarking that the position on the face seemed to accord with 

 the area exposed by the peasant headdress. His studies were reported at 

 length in 1907. There were also extensive reviews by Hyde in 1906 and 

 by Bellini in 1909. Four principal lines of evidence supporting the idea 

 that sunlight is a cause of cancer of the skin of man were based on these 

 early clinical observations: (1) cancer of the skin occurs principally on 

 parts exposed to sunlight; (2) cancer of the skin is more prevalent in out- 

 door workers than in sedentary workers; (3) the incidence of cancer of the 

 skin is greater in regions of the earth that receive the greatest insolation; 

 and (4) cancer of the skin occurs more often in light-complexioned persons 

 than in dark. At the time these arguments were outlined, basic informa- 

 tion that was essential to support them was lacking, and this information 

 could not be supplied until experimental studies on animals had been 

 initiated. The various lines of evidence will be examined in some detail 

 in this chapter. 



The laboratory studies have also provided a tool for studying the 

 process of carcinogenesis by ultraviolet radiation. The successes and 

 difficulties encountered will be discussed a little later. But the experi- 

 ments themselves, what they show, and what their limitations are must be 

 discussed before either the mechanism of carcinogenesis or the etiology of 

 human cutaneous cancer is considered. 



EXPERIMENTAL STUDIES 



The Carcinogenic Wave Lengths. Obviously, one of the first things to be 

 done in the laboratory was to delimit the wave lengths that induce the 

 cancers. The first attempt to do this was made by the late A. H. KotTo, 

 whose findings appear extensively and rather diffusely described in a 

 bulletin publi.shed in Spanish from his institute in Buenos Aires. Many 

 of th(> more important ones are, however, briefly summarized in an article 

 published in French in 1934. Roffo exposed rats to mercury arc radiation 

 passing through various colored glasses. These filters are not accurately 

 described, but one which is designated as "verre transparent" may l)e 

 assumed to be common window glass. Whereas rats exposed directly to 

 the mercury arc radiation developed cancers, those protected by this glass, 



