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Precursors of Melanoma

Part I: Naevi and melanoma. Which naevi matter? It has been known for some time that certain melanocytic naevi have premelanoma potential. The question arises: Which of these naevi have the potential? Identification and appropriate management of such precursors could markedly reduce the incidence of and mortality from melanoma. This article deals with the most common precursors of malignant melanoma – a peculiar subtype of acquired melanocytic naevi, atypical moles (dysplastic naevi), giant congenital melanocytic naevi and lentigo maligna. Melanocytic naevi are very common, virtually everyone has some. They may be acquired or congenital. The acquired kind are not present at birth and develop mostly in the first twenty years of life, occasionally even later. Melanocytic naevi are composed of melanocytes which either did not mature or stopped in their migration towards the basal cell layer of the epidermis during embryonic development (blue naevi). An acquired melanocytic naevus appears when a group of immature melanocytes starts to proliferate at the dermo-epidermal junction (junctional naevi). After a period of horizontal growth along the dermoepidermal junction, some melanocytes grow down into the dermis (compound naevi). Finally, all melanocytes come to be located in the dermis (intradermal naevi). Different melanocytic naevi may be at different phases of their development in the same individual. Junctional naevi are usually flat and uniformly pigmented. Compound naevi are usually slightly elevated, a bit lighter in colour and often mottled or stippled. Intradermal naevi are usually dome-shaped or even pedunculated, pale or very lightly pigmented. Blue naevi (Fig 1) have a characteristic slate-blue colour resulting from the deep intradermal location of pigment producing melanocytes. (Fig 1) Congenital melanocytic naevi are found in between 1% and 6% of newborn infants. These are usually bigger than acquired naevi. Congenital naevi may vary in size from a few to many centimeters. Giant congenital naevi are those greater than 20cm in size or, by another definition, so large that they cannot be removed by simple excision and primary closure using adjacent tissue in a single surgical procedure. (Fig 2) Precursors of Melanoma