Patterns of invasive melanoma in the Connecticut Tumor Registry. Is the long-term increase real?

The decades-long increase in incidence rates for melanoma has been ascribed to artifactual changes in case ascertainment rather than to true changes in disease risk. In this study, population-based incidence data for invasive cutaneous malignant melanoma from the Connecticut Tumor Registry were categorized into seven age groups and four time periods to examine the pattern of change over four decades. Analyses of age, period, and cohort variables focused on the curvature components, which are estimable functions. Statistical modeling demonstrated the following: (1) incidence rates have increased by birth cohort in both sexes with no requirement for a period variable, regardless of whether data are examined by 10-year, 5-year, or 1-year intervals of diagnosis; (2) this pattern in incidence rates differed from the patterns of change in the two indices of case ascertainment, the proportion of cases confirmed microscopically and the proportion of cases in localized stage, both of which exhibited changes by period of diagnosis rather than by birth cohort; and (3) adjustment for these two indices caused a downward bend in the cohort curve for females but not for males. The results suggested that much of the observed increase for this tumor was real.