Squamous cell carcinoma (SCC) is the second most common type of skin cancer and accounts for approximately 20% of cutaneous malignancies. Although most cancer registries do not include information on the incidence of nonmelanoma skin cancer, annual incidence estimates range from 1 million to 3.5 million cases in the United States.[1,2]
Mortality is rare from this cancer; however, the morbidity and costs associated with its treatment are considerable.
Risk Factors for Squamous Cell Carcinoma
Sun exposure is the major known environmental factor associated with the development of skin cancer of all types; however, different patterns of sun exposure are associated with each major type of skin cancer. (Refer to the Sun exposure section in the Basal Cell Carcinoma section of this summary for more information.) This section focuses on sun exposure and increased risk of cutaneous SCC.
Unlike basal cell carcinoma (BCC), SCC is associated with chronic rather than intermittent intense exposure to ultraviolet (UV) radiation. The characteristic pattern of sun exposure linked with SCC is occupational exposure. A case-control study in southern Europe showed increased risk of SCC when lifetime sun exposure exceeded 70,000 hours. People whose lifetime sun exposure equaled or exceeded 200,000 hours had an odds ratio 8 to 9 times that of the reference group. A Canadian case-control study did not find an association between cumulative lifetime sun exposure and SCC; however, sun exposure within 10 years and occupational exposure were found to be risk factors.
Other radiation exposure
In addition to environmental radiation, exposure to therapeutic radiation is another risk factor for squamous cell carcinoma. Individuals with skin disorders treated with psoralen and ultraviolet-A radiation (PUVA) had a threefold to sixfold increase in SCC. This effect appears to be dose-dependent, as only 7% of individuals who underwent fewer than 200 treatments had SCC, compared with more than 50% of those who underwent more than 400 treatments. Therapeutic use of ultraviolet-B (UVB) radiation has also been shown to cause a mild increase in SCC (adjusted incidence rate ratio, 1.37). Devices such as tanning beds also emit UV radiation and have been associated with increased SCC risk, with a reported odds ratio (OR) of 2.5 (95% confidence interval [CI], 1.7-3.8).
Investigation into the effect of ionizing radiation on SCC carcinogenesis has yielded conflicting results. One population-based case-control study found that patients who had undergone therapeutic radiation had an increased risk of SCC at the site of previous radiation (OR = 2.94) as compared with individuals who had not undergone radiation treatments. Cohort studies of radiology technicians, atomic-bomb survivors, and survivors of childhood cancers have not shown an increased risk of SCC, although the incidence of BCC was increased in all of these populations.[11,12,13] For those who develop SCC at previously radiated sites that are not sun-exposed, the latent period appears to be quite long; these cancers may be diagnosed years or even decades after the radiation exposure.