Incidence of basal cell carcinoma multiplicity and detailed anatomic distribution: longitudinal study of an Australian population.

A proportion of individuals are affected multiple times by basal cell carcinoma (BCC), but the rate and extent to which this occurs is unknown. We therefore prospectively estimated BCC incidence in a subtropical Australian population, focusing on the rate at which persons develop multiple primary BCCs and the precise anatomic sites of BCC occurrence. Between 1997 and 2006, 663 BCCs were confirmed in 301 of 1,337 participants in the population-based Nambour Skin Cancer Study. The incidence of persons affected multiple times by primary BCC was 705 per 100,000 person years compared to an incidence rate of people singly affected of 935 per 100,000 person years. Among the multiply and singly affected alike, site-specific BCC incidence rates were far highest on facial subsites, followed by upper limbs, trunk, and then lower limbs. We conclude that actual BCC tumor burden is much greater in the population than is apparent from normal incidence rates. Anatomic distribution of BCC is consistent with general levels of sun exposure across body sites.