Geographical clustering of anal cancer incidence in Australia

Homosexual men are at an increased risk of anal cancer. We aimed to establish the burden of anal squamous cell carcinoma (SCC) in those parts of Australia where homosexual men are most likely to live.

Data on the proportion of homosexual male residents were obtained from published estimates. Men were categorised into three postcode groups by prevalence of men reporting homosexual identity. Male population data in age groups were extracted for each postcode group and analyses of cancer incidence were performed by postcode group. The analyses were restricted to 2000-2005.

Eight postcodes had populations where more than 10% of males reported homosexual identity (high prevalence) and 4-10% of men reported homosexual activity in a further 19 postcodes (medium prevalence). From 2000 to 2005, the average annual age-standardised incidence rates of anal SCC in males was 7.61 per 100000 (95% confidence interval (CI): 4.68-10.55) and 2.21 per 100000 (95% CI: 1.05-3.37) in high and medium prevalence postcodes, respectively. The corresponding incidence rate ratios compared with low prevalence postcodes (less than 4% of males reported homosexual identity) were 9.6 (95% CI: 6.6-14.1) for the high prevalence and 2.4 (95% CI: 1.4-4.1) for the medium prevalence postcodes.

A substantial concentration of the burden of anal cancer occurred among areas where large proportions of homosexual men reside. These results should guide the prioritisation of health service investment in anal cancer treatment and prevention to appropriate geographical areas.