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2. Potential Precursors of Melanoma
Revised January 2008
While melanomas can occur in childhood, they are rare. Clinicians should be alert, as melanomas in childhood can be confused with a benign Spitz nevus. A careful review of pathology may spare disfiguring surgery.
1. Common Acquired Melanocytic Nevi
Evidence for the role of common acquired nevi as precursors of melanoma is at present morphological and depends on the histological identification of benign nevus cells in contact with melanoma cells. The frequency with which benign acquired melanocytic nevi develop into melanoma is extremely low given the average number of nevi per person and the relatively low rate of melanoma around the world.
2. Atypical Nevi
Atypical nevi were first recognized as an autosomal dominant condition in the setting of hereditary melanoma. It is now recognized that atypical nevi may also appear in a non-hereditary setting. In both these situations, atypical nevi are markers for individuals at increased risk to develop melanoma. The clinical features of atypical nevi are generally: - >6 mm; colour - multiple shades of brown, or red-brown; shape - irregular; surface - pebbly or centrally raised; border - poorly demarcated. The diagnosis of atypical nevi should be based on clinical and histologic criteria. However, the correlation between clinical and pathologic diagnosis is rather poor. This could be in part due to the focal nature of the pathology which may be missed on routine histologic sections. The problems in diagnosis suggest clinical diagnostic criteria need to be refined.
Atypical nevi are potential precursors of melanoma. This has been documented clinically with photographs. Histologically, atypical nevi have been found in continuity with melanoma in a number of reported series. In addition, atypical nevi are markers for increased melanoma risk on normal skin on the affected individuals. Patients with atypical nevi need regular skin surveillance.
See: Management of atypical nevi.
3. Congenital Melanocytic Nevi
Congenital melanocytic nevi (CMN) are arbitrarily classified into 3 categories according to size: large, >20 cm in diameter; medium, between 1.5 cm and 20 cm, and small, <1.5 cm in diameter. The incidence of CMN is about 1%. CMN carry an increased risk for developing melanoma. For large CMN, the lifetime risk of melanoma transformation is about 6-10%. The magnitude of melanoma risk for small and medium CMN has not been conclusively determined.
CMN, which are usually raised and hairy, should be distinguished from cafe-au-lait patches which are flat, light brown patches. Cafe-au-lait patches do not have malignant potential.
See: Management of congenital melanocytic nevi.
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