Exclusive development of a single type of keratinocyte skin cancer: evidence from an Australian population-based cohort study.

Whether susceptible people develop both basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) or one type exclusively during life is unknown. We investigated this in an Australian community cohort of 1,191 adults aged 25-75 years by recording all new BCCs and SCCs for 16 years in people with no previous keratinocyte cancer. Among those who developed multiple skin cancers, age- and sex-specific incidence rates per 100,000 were calculated for those who developed BCC exclusively, SCC exclusively, or BCC and SCC. Corresponding relative risks (and 95% confidence intervals) were estimated by Poisson regression. During follow-up, 116 people developed multiple keratinocyte cancers: 65 (56%) developed BCC exclusively (range 2-8 per person); 18 (16%) developed SCC exclusively (2-5 per person); and 28% developed both types. Of the 116, 88 had a BCC first, of whom 74% subsequently developed only BCCs, and 28 had SCC first, of whom 64% subsequently developed only SCCs. Incidence rates did not differ by sex in the BCC-only, SCC-only, or mixed groups, but they increased significantly with age especially in the SCC-only group. These findings suggest that the majority of people are prone to develop one type rather than a mix of keratinocyte cancers.