Left-sided excess of invasive cutaneous melanoma in six countries.

To investigate the laterality of cutaneous melanoma (ICD-10 C43), we obtained data spanning the period of diagnosis 1998-2003 from six population-based cancer registries: New South Wales, Australia; England; Finland; The Netherlands; Scotland; and the Surveillance Epidemiology and End Results (SEER) Program of the United States of America (restricted to 'White' race category only). For cases with laterality recorded, the overall ratios of left- to right-sided tumours were calculated. We found that left to right ratios were consistently greater than 1.00, ranging from 1.08 (New South Wales, Australia and US SEER 'White') to 1.18 (Scotland), with an overall ratio for all registries combined of 1.10 (95% confidence interval 1.08-1.11). There were no statistically significant differences by sex or age group for all sub-sites combined, or for upper limb or lower limb melanomas. The excess of left-sided tumours seems unlikely to be explained by chance or recording bias. The most likely explanations would appear to be either differences in sun exposure and/or asymmetry of melanocyte distribution or characteristics arising at the time of embryological development.