Higher melanoma incidence in coastal versus inland counties in California

The incidence of melanoma is increasing and there is significant variation by geographical location between and within countries. We sought to determine the incidence of melanoma in coastal versus inland counties in California. Data on melanoma incidence were obtained for 2000-2009 from the Surveillance, Epidemiology, and End Results program of the National Cancer Institute. Incidences for melanoma in situ and invasive melanoma for major racial and ethnic groups for coastal and inland counties were analyzed using multivariable Poisson regression, with adjustment for socioeconomic factors (income, education), ultraviolet index, and latitude. Further analyses were carried out for the non-Hispanic white population through stratification of in-situ versus invasive melanoma, age, thickness, and anatomic distribution. The incidence of melanoma in situ is greater in coastal counties of California than inland counties (incidence rate ratio 1.23, 95% confidence interval 1.04-1.47) after adjusting for socioeconomic factors, ultraviolet index, and latitude. In non-Hispanic whites, this difference is significant for in-situ and thin (≤ 1.00 mm) melanomas, but not for melanomas of greater thickness. In melanoma in situ and thin melanomas in non-Hispanic whites, the incidence is greater in coastal versus inland counties. Causes may include differences in exposures, differences in detection, or artifacts such as residual confounding. Our study highlights the need for further research in identifying and addressing these differences.