Objectives: To compare cancer risks after in situ and invasive squamous cell carcinoma (SCC) of the skin and to determine whether these 2 forms of cancer differ in prognostic significance. Patients: Subsequent events after in situ and invasive SCC were studied in the Swedish Family-Cancer Database, in which cancer data were obtained from the Swedish Cancer Registry from 1958 to 1996. Among 22 293 patients with in situ SCC, 3940 had first invasive cancer; among 17637 patients with invasive SCC, 3624 had a second occurrence of cancer. Main Outcome Measure: Standardized incidence ratios (SIRs), ratios of the observed to expected number of cases, served as a measure of relative risk. For overall risks, cases diagnosed within the first year of follow-up were omitted. Results: The median age of onset was 72 to 73 years for in situ and invasive SCC, respectively. Standardized incidence ratios of all cancers were increased after in situ SCC (men-women, 1.5:1.3) and invasive SCC (men- women, 1.9:1.5). The subsequent occurrences of cancer and their SIRs were similar after in situ and invasive SCC, with skin cancer showing the highest SIR of 6.4:10.0. Among discordant cancers, increased SIRs were recorded for melanoma and a group of malignant neoplasms observed in patients with immunosuppression, including lymphoma and oral cancers. Subsequent cancers in the salivary glands and nasal cavity also showed increased SIRs, particularly after invasive SCC. Conclusion: Risks of subsequent cancers, including skin cancer, melanoma, and internal cancers, showed similar patterns in patients with in situ and invasive SCC, suggesting that the 2 groups have a similar susceptibility to cancer.
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