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.

Although sun exposure is known to be associated with basal cell carcinoma (BCC), it is not known what determines multiple occurrences of BCCs among sporadically affected individuals or why BCCs develop on uncommonly sun-exposed body sites like the trunk. In a prospective community-based skin cancer study in Queensland, Australia, we studied all participants who experienced a histologically confirmed BCC from 1992 to 2007. Sun exposure history was monitored, and dermatologists documented phenotype at baseline and signs of photodamage over the study period. Anatomic sites of all incident BCCs were recorded. Odds ratios (ORs) and 95% confidence intervals (95% CIs) were calculated using logistic regression. Of 401 participants who developed a new BCC during the 16 years of follow-up, 232 (58%) developed more than 1. Male sex (OR 5 2.5, 95% CI 1.5-5.3) and age 60 or over (OR 5 4.2, 95% CI 1.5-11.8) but not skin type were associated with highest BCC counts among those affected. Participants with high numbers of solar keratoses were most likely to experience the highest BCC counts overall (OR 5 4.3, 95% CI 1.4-13.5). Moreover, occurrences of BCC on the trunk (OR 5 3.3, 95% CI 1.4-7.6) and on the limbs (OR 5 3.7, 95% CI 2.0-7.0) were strongly associated with high numbers of solar keratoses on these sites, respectively. Among those newly affected by BCC, chronic cutaneous sun damage predicts those who will be affected by more than 1 BCC, while chronic sun damage on the trunk and limbs predicts BCC occurrence on the trunk and limbs, respectively.Basal cell carcinoma (BCC), the commonest cancer, occurs more than once in a high proportion of individuals sporadically affected. In North American study populations with relatively low incidence of BCC, 1,2 2 prospective cohort studies reported that around 34% of affected people developed multiple BCCs within 5 years of follow-up. 3,4 In a high-incidence Australian population, nearly 50% of affected people developed multiple BCCs over 10 years. 5 Hence, it is not only the number of people affected but also the multiple tumors they develop that underlies the substantial health expenditure attributed to BCC treatment in North America, 6 Europe 7 and Australia. 8 A limited number of previous studies in selected clinical populations have suggested that multiple BCC occurrence is associated with increasing age, 9 male sex 10 and fair skin, 11,12 but little is known about the importance of other pigmentary and environmental factors including sun exposure history and photodamage.Another unanswered question about BCC is why a sizable proportion occurs on sites that are often sun protected like the trunk and limbs, given BCC's strong association with sun exposure. 13 Some have suggested different mechanisms of BCC pathogenesis on different sites because head/neck BCCs appeared more strongly associated with a sun-sensitive phenotype than trunk BCCs. 12,14,15 Also, people initially affected by trunk BCC were reported to be affected more often subsequently on the trunk than people with an i...