Thick malignant melanoma is a long-term problem

Aims. To investigate changes in incidence of in-situ, 'thin' (<0.76 mm) and 'thick' (>0.76 mm) melanoma. To explore the relationship of melanoma depth with the patient's age at diagnosis. Methods. A retrospective case series of melanoma was taken from pathology r ecords in Palmerston North between 1983 and 1994. The denominator populatio n was estimated, for each year, from published 1986 and 1991 census figures . Results. Over the 12 years, the diagnostic rate of in-situ and invasive mel anomas increased annually by 16% and 5%, respectively (Poisson regression, p<0.0001). This analysis did not show a difference between the overall rate of increase in 'thin' and 'thick' invasive melanomas. After excluding Hutc hinson's melanoma, the mean age at diagnosis for 'thin' and 'thick' melanom as was 49.0 and 55.4 years, respectively (p<0.0001); the peak number of 'th in' and 'thick' melanomas were in the 30-39 and 60-69 age groups, respectiv ely. Conclusions. There is evidence for progression from 'thin' to 'thick' melan oma over a 6-to 30-year period. Poor prognosis 'thick' melanomas are curren tly increasing. Providing removal of 'thin' melanomas is sustained, a decli ne in 'thick' melanomas is expected but may take decades.