The systematic relationship between melanomas diagnosed in situ and when invasive

The Surveillance, Epidemiology and End Results (SEER) Program of the US National Cancer Institute has provided population-based, histologically confirmed incidence data for melanomas restricted to the epidermis (in situ) and invasive melanomas for representative areas of the US for the years 1975 to 1997. The incidence of in situ melanomas has increased more rapidly than that for invasive melanoma, but both appear to conform to the birth-cohort pattern. As both changed from year to year, the incidence of melanoma in situ and invasive melanoma at the time of diagnosis were linked by simple proportional relationships. The ratio of in situ to invasive tumours was similar in males and females, suggesting that the generally better female prognosis develops once the tumour has spread into the dermis. Across the age groups, the patients with in situ tumours were older than those with invasive tumours. This suggests slower growth, while histological confirmation of the diagnoses and the precise linkage between the incidence rates for in situ and invasive melanomas implies a close biological connection between tumours that progress and those that do not. Study of the in situ tumours could reveal what was holding them back.