New primary cancers after squamous cell skin cancer.

In a search for clues to the origin of squamous cell skin cancer (SCC), the authors investigated the pattern of new cancers in a cohort of 5,100 SCC patients whose tumors were diagnosed during the years 1978-1989 and recorded in the Danish Cancer Registry. Subsequent cancer experiences in SCC patients were compared with the cancer incidence in the Danish population using ratios of observed cancers to expected cancers as a measure of the relative risk. Overall, patients with SCC were at increased risk of new malignancies (relative risk (RR) = 1.6, 95% confidence interval (CI) 1.5-1.7). Significantly elevated risks were found for cancers of the respiratory organs (RR = 1.7, 95% CI 1.4-2.0); cancers of the lip, buccal cavity, and pharynx (RR = 3.1, 95% CI 2.1-4.5); non-Hodgkin's lymphoma (RR = 2.3, 95% CI 1.4-3.5); leukemia (RR = 2.5, 95% CI 1.7-3.5); malignant melanoma (RR = 2.6, 95% CI 1.5-4.3); and cancer of the small intestine in men (RR = 4.1, 95% CI 1.1-10.6). The risk of new cancers (other than nonmelanoma skin cancers) was higher in patients diagnosed with SCC before the age of 60 years (RR = 1.9, 95% CI 1.5-2.5) than in those diagnosed with SCC at or after that age (RR = 1.3, 95% CI 1.2-1.4). The data confirmed previous strong associations between SCC and malignant melanoma and cancers of the major salivary glands. A previously undocumented significant excess of smoking-related cancers was observed after an SCC diagnosis, suggesting that smoking may be involved in the development of SCC. Since a variety of other squamous cell cancers have already been linked to smoking, the authors speculate that some general effect of smoking might act on all human squamous epithelia. The observed significant associations with lymphoma and leukemia and the high risk of subsequent malignancies in young SCC patients merit further attention.