Histogenesis of malignant melanoma with an adjacent component of the superficial spreading type

There has been a world-wide exponential increase in the incidence of thin malignant melanoma. At the Sydney Melanoma Unit, the proportion of patients diagnosed as having superficial spreading melanoma has more than doubled from 33% prior to 1960 to 78% during 1980-83. A study was made of the non-invasive component of malignant melanoma with an adjacent non-invasive component of the superficial spreading type in an attempt to elucidate the pathogenetic mechanisms involved in these changing trends. In this study on 723 cases of melanoma with a superficial spreading component, there was evidence that 39% originated in a precursor lesion. In the remaining 61%, the adjacent superficial spreading component consisted of melanoma in situ, suggesting that these were melanomas from the beginning. The latter lesions were thinner and had a lower degree of mitotic activity than melanomas commencing in a precursor lesion. Despite the large increase in incidence of superficial spreading melanomas and the shift to thinner lesions over time, there appeared to be no difference in the proportion of lesions commencing de novo to those commencing in a precursor lesion. This suggests that the precursor lesion may be of genetic origin.