Section 12: Melanoma Precursors and Primary Cutaneous Melanoma

PRECURSORS OF CUTANEOUS MELANOMA

Precursors of melanoma are lesions that are benign per se but have the potential of turning malignant and thus giving rise to melanoma. Two such entities are recognized: (1) dysplastic melanocytic nevi and (2) congenital melanocytic nevi.

DYSPLASTIC MELANOCYTIC NEVUS (DN) ICD-10: D48–54

• Dysplastic nevi (DN) are a special type of acquired, circumscribed, pigmented lesions that represent disordered proliferations of variably atypical melanocytes.

• DN arise de novo or as part of a melanocytic nevus.

• DN are clinically distinctive from common acquired nevi: larger and more variegated in color, asymmetric in outline, and with irregular borders; they also have characteristic histologic features.

• DN are regarded as potential precursors of superficial spreading melanoma (SSM) and also as markers of persons at risk for developing primary melanoma of the skin, either within the DN or on normal skin.

• DN occur either sporadically or in the context of the familial DN syndrome: Kindreds with familial multiple DN and melanomas (formerly FAMMM, or B-K mole syndrome).

• Synonym: atypical melanocytic nevus.

EPIDEMIOLOGY

AGE OF ONSET Children and adults.

PREVALENCE Dysplastic nevi (DN) are present in 2% to 10% of White people. They occur in almost every patient with familial cutaneous melanoma and in 30% to 50% of patients with sporadic nonfamilial primary melanomas of the skin.

SEX Equal in males and females.

RACE White persons. Rarely seen in the Asian, Brown, and Black population.

TRANSMISSION Autosomal dominant.

PATHOGENESIS

Multiple loci have been implicated in familial melanoma/DN syndrome, and it is likely that DN is a complex heterogeneous trait. It is assumed that an abnormal clone of melanocytes can be activated by exposure to sunlight. Immunosuppressed patients (renal transplantation) with DN have a higher incidence of melanoma. DN favor the exposed areas of the skin, particularly intermittently sun exposed (e.g., back) and this may be related to the degree of sun exposure. However, DN may also occur in completely covered areas.

CLINICAL MANIFESTATION

DURATION OF LESIONS DN usually arises later in childhood than common acquired melanocytic nevus (MN), which first appear in late childhood, just before puberty. New lesions continue to develop over many years in affected persons; in contrast, common acquired MN do not appear after middle age and disappear entirely in older persons. DN are thought not to undergo spontaneous regression as much as common acquired MN. Also, whereas common MN are usually in a roughly comparable stage of development in a given body region (e.g., junctional, compound, dermal), DN appear “out of step,” e.g., a mix of large and small, flat and raised, tan, and very dark lesions (Fig. 12-1).