Cancer of the skin usually occurs in those well past middle life.
The two sexes are almost equally affected, except in cancer of the mouth, which is very rare in women.
The native American Indian rarely suffers from cancer of the skin. The negro only exceptionally suffers from basal-celled cancer, but does have the spinocelled type about as frequently as the white man. In the fifteen cases of cancer of the skin and mucous membrane that the author has recently seen among negroes there was but one rodent ulcer; all of the other lesions were microscopically proven to be of a more virulent type. The lymphomata are likewise rare.
The multiple, idiopathic, hemorrhagic sarcoma of Kaposi is most frequent in Hebrews.
Judging from clinical data alone, heredity seems to play no part in the production of cancer of the skin. Some of the multiple benign growths are, however, often found in members of the same family.
It would be highly improper in dealing with cancer not to speak of the important work done by Maud Slye.18 She has shown that in mice the tendency to develop cancer is transmitted from generation to generation in exact accord with the laws of heredity, so that it can be bred in and out of strains of mice at will. It is not cancer itself that is transmitted, but the tendency of the cells to produce cancer under suitable conditions. She states: "The elimination, as far as possible, of all forms of overirritation of the tissues of an individual of high cancer ancestry should go far to eliminate the provocation of cancer; and the eugenic control of matings, so that cancer shall at least not be potential on both sides of the hybrid cross, ought to eventuate in a considerable decrease in the frequency of human cancer".
*Rous and Murphy: Jour. Exp. Med., 1913, xvii, 219. 17Smith: Washington Med. Annals, 1913, xii, 1. 18 Slye: Jour. Med. Research, 1914, 1915.
By trauma is meant not so much one injury as a chronic irritation or a series of injuries. Cancer of the skin is very apt to originate in a spot that has been more or less continuously subjected to irritation, or upon an abnormal part of the skin, such as a wart or mole. The melanotic carcinomata spring from congenital, or more rarely acquired, moles that have been irritated. The more malignant types of cutaneous carcinomata are apt to spring from the scars of old burns, or of some preexisting dermatosis.
Hyde* has clearly shown that those who are the most exposed to sunlight are the most apt to suffer from basal-celled carcinomata, the malignant growth usually being preceded by a keratosis. More recently Lawrence* has called attention to the prevalence of keratoses and cancer in the dry and sunny regions of Australia, and entirely independently reaches the same conclusions as does Hyde. It is a well-known fact that in xeroderma pigmentosum the lesions are made much worse by exposure to the actinic rays of the sun. Unna has described a condition known as "sailor's skin,,, in which seamen more especially, but also those exposed to much light, suffer from multiple cancers of the more benign variety. Of course, cancer is especially apt to develop upon chronic x-ray dermatoses. It may be clearly stated, therefore, that exposure to actinic rays certainly leads to the formation of cancer of the skin.
Schamberg* has recently contributed an excellent chapter to the subject of cancer due to. certain occupations, especially chimney sweeps, and paraffin and tar workers. These substances undoubtedly cause keratoses upon the skin, and these keratoses may later become malignant. In addition to reporting several cases of his own, Schamberg has thoroughly reviewed the literature and collected all of the reported cases. It would seem that cancer, usually of a mild form, is very common among those forced to handle the crude tar, and that in nearly every factory a number of cases can be found. The lesions are usually situated upon the hands, and are associated with a marked folliculitis. In chimney sweeps the disease generally appears upon the scrotum, due to the collection of the soot in the skin ruga? in this region, and, while at first comparatively benign, yet quite frequently leads to death. Cancer is also reported in gardeners who handle soot in the course of their work.
*Hyde: Amer. Jour. Med. Scien., 1906, cxxxi, 1.
*Lawrence: Trans, of 7th Internat. Congr. Derm, and Syph., Rome, 1912, 1219. "Schamberg: Jour. Cut. Dis., 1910, xxviii, 644.
The etiology of cancer of the skin is still sub judice, but we do know that cancer frequently develops upon keratoses caused by various external irritants, such as tar. light, or the internal administration of arsenic. Also, cancer is apt to develop upon other lesions of the skin, these being fully considered in the next chapter. The work of Smith in showing that a disease in plants that is certainly analogous to cancer is due to bacteria, the work of Sous and Murphy in demonstrating that chicken sarcoma can be inoculated from a filtrate passed through a Berkfelt filter. Ehrlieh s22 demonstrations that mouse cancer is rendered more virulent by being transmitted through several generations, and Gaylord "s23 experiences with infected cages, not to mention certain of Behla V* statistics, would seem to show that cancer is probably due to an infecting organism of some sort. It is well known that a riotous proliferation of cells can be caused by external irritants, and the fact that cancer so frequently develops upon the site of an old injury or of a more recent break in the skin may simply mean that here is the chance for the infecting organism to gain foothold. The long periods of latency before a metastasis manifests itself is certainly in harmony with this theory. Against it speaks strongly the rarity of "cancer a deux".
*Ehrlich: Zeitschr. f. AerxlL Fortbild.. 7.
*Gaylord: Jour. Am. Med. Assn.. IJK'7. xlviii. 15. * Behla: Deut. med. Wchr.schr.. 1*01. xxvii. 4~.