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 nevomelanocytic nevi, and (2) congenital nevomelanocytic nevi.
Dysplastic Melanocytic Nevus
ICD-9 : 238.2 • ICD-10 : D48-5
- Dysplastic melanocytic 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 compound melanocytic nevus.
- DN are clinically distinctive from common acquired nevi: larger and more variegated in color, asymmetric in outline, irregular borders; they also have characteristic histologic features.
- DN are regarded as potential precursors of superficial spreading melanoma and also as markers of persons at risk for developing primary malignant 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).
- Synonyms: atypical melanocytic nevus, dark nevus
DN are present in 5% of the general white population. They occur in almost every patient with familial cutaneous melanoma and in 30–50% of patients with sporadic nonfamilial primary melanomas of the skin.
Equal in males and females.
White persons. Data on persons with brown or black skin are not available; DN are rarely seen in the Japanese population.
Multiple loci have been implicated in familial melanoma/DN syndrome. It is assumed than 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, and this may be related to the degree of sun exposure.
DN usually arise later in childhood than common acquired nevomelanocytic nevi (NMN), appearing first in late childhood, just before puberty. New lesions continue to develop over many years in affected persons; in contrast, common acquired NMN do not appear after middle age and disappear entirely in older persons. Also, whereas common NMN 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. 12-1A).