Jospeh Scotto, M.S.*
|Nonmelanoma skin cancer is the most common cancer among
whites in the United States. Because most nonmelanoma skin cancer
patients are treated in doctors' offices, population-based estimates
of skin cancer incidence are fairly difficult to obtain. More than
600,000 new cases of nonmelanoma skin cancer may occur in the United
States each year, and this number is rising (Glass and Hoover, 1989;
NIH Consensus Development Conference, 1989). The fatality rate from
nonmelanoma skin cancer is less than 1 percent (Marks, 1988).
The incidence of nonmelanoma skin cancer varies directly with exposure to ultraviolet (UV) light from the sun and, indirectly, with the degree of skin pigmentation. Thus, nonmelanoma skin cancer is most common among fair-skinned whites who live in sunny locales. The highest rates in the past have been recorded among Caucasians in South Africa and Australia (Marks et al., 1989). Ireland, despite its rain and mist, has had a high incidence because of the susceptibility of persons of Celtic ancestry (Urbach, 1971).
Nonmelanoma skin cancer occurs less often in Hispanics and Orientals, and least often among blacks. In the United States, for example, a National Cancer Institute survey (Scotto, 1983) showed that the age-adjusted incidence rate was only 3.4 per 100,000 among blacks, or about 80 times less than the rate observed among whites.
Most nonmelanoma skin cancers are of two types--squamous cell carcinoma and basal cell carcinoma. The basal cell type is more common, but the squamous cell type is more invasive, and may account for about three-fourths of all deaths from nonmelanoma skin cancer (Dunn et al., 1965).
Several studies have shown that basal cell skin cancer occurs about one and one-half to two times more often in white men than in white women, and squamous cell skin cancer occurs two to three times more often in men. Both types occur most often on the face, head and neck (about 80 percent for both types combined). Women have higher rates than men for both types of cancers on the legs (Scotto, 1982), consistent with their greater sun exposure at this anatomical site. White men have more squamous cell carcinoma of the lip, in line with their risks from tobacco and outdoor work (Lindqvist, 1979).
Skin cancer incidence in the United States is positively correlated with annual dosages of surface solar ultraviolet radiation (UVB) received at each geographic location (Fears and Scotto, 1983). The direct relationship is most clearly seen with squamous cell carcinoma of the skin (Scotto and Fraumeni, 1982), and varies according to cell type. A 1 percent increase in UVB exposure may result in incidence increases of 4, 2, and 1 percent for squamous cell, basal cell, and malignant melanoma of the skin, respectively. This is consistent with the evidence that factors other than sunlight also contribute to the development of melanoma (Greene and Fraumeni, 1979).
There are other risk factors for nonmelanoma skin cancers. They were, for example, the first type of cancer related to ionizing radiation exposure, with reports as early as 1902 among radiation workers (Martin, 1970). Other studies have shown an excess risk associated with radiotherapy for a number of diseases (Matanoski, 1975). Excess risks have also been noted among radiologists and uranium miners (Sevcova, 1978).
Exposure to a number of chemicals may also induce skin cancer in animals, particularly squamous cell carcinoma. Epidemiologic studies substantiate their associated risk in humans. Polycyclic aromatic hydrocarbons induce cancers in animals and are found in coal tars, pitch, asphalt, soot, creosote, and lubricating and cutting oils (Kipling, 1976). Skin and other forms of cancer have been found in various worker groups exposed to these substances. Though Orientals rarely develop sun-induced skin cancer, arsenic exposure (e.g., from artesian well water) may result in excess risk for skin cancer (EPA, 1988).
Studies have shown an excess risk of skin cancer among psoriasis patients treated with crude tar ointments and UVA, i.e., ultraviolet wavelengths of 330 to 400 nm, (Stern, 1980), and there has been increased concern about the possible hazards of other photosensitizers found in tanning aids, cosmetics, and medicines (NIH Consensus Development Conference, 1989).
Squamous cell skin cancer has also been observed as a complication of tropical ulcers, burns, scars, and chronic infections and wounds (Malik et al., 1974), chiefly among dark-skinned populations in Africa and Asia, but recent studies of black Americans have indicated that burn scars or chronic infections may predispose them to skin cancer also (Fleming, 1975). Actinic keratoses--brownish, hardened areas on skin exposed to excess sunlight--are considered precursor lesions for squamous cell skin cancer (Marks, 1988). Individuals with several rare hereditary diseases, including multiple basal cell carcinoma syndrome, xeroderma pigmentosum, and albinism, are also at heightened risk of developing skin cancer (Kraemer, 1984).
Avoiding overexposure to sunlight is the best way to prevent nonmelanoma skin cancer. In addition to natural sunlight, it is also important to avoid unnecessary X-rays and ultraviolet light exposure from artificial sources such as sunlamps and tanning booths.