SECTION 9: BENIGN NEOPLASMS AND HYPERPLASIAS

DISORDERS OF MELANOCYTES

ACQUIRED MELANOCYTIC NEVI (MN) ICD-10: D22.L10

• MN, commonly called moles, are very common, small (<1 cm), circumscribed, acquired pigmented macules, papules, or nodules.

• Composed of groups of melanocytic nevus cells located in the epidermis, dermis, and, rarely, the subcutaneous tissue.

• They are benign, acquired tumors arising as nevus cell clusters at the dermal–epidermal junction (junctional MN), invading the papillary dermis (compound MN), and ending their life cycle as dermal MN with nevus cells located exclusively in the dermis where, with progressive age, there will be fibrosis.

EPIDEMIOLOGY AND ETIOLOGY

One of the most common acquired new growths in Caucasians (most adults have about 20 nevi), less common in African Americans or darker pigmented persons; sometimes absent in persons with red hair and marked freckling (skin phototype I).

RACE African Americans and Asians have more nevi on the palms, soles, and nail beds.

HEREDITY Common acquired MN occur in family clusters. Dysplastic melanocytic nevi (DN) (see Section 12), which are putative precursor lesions of malignant melanoma, are different from MN. They occur in virtually every patient with familial cutaneous melanoma and in 30 to 50% of patients with sporadic nonfamilial primary melanoma.

SUN EXPOSURE A factor in the induction of nevi on the exposed areas.

SIGNIFICANCE Risk of melanoma is related to the numbers of MN and to DN. In the latter, even if only a few lesions are present.

CLINICAL MANIFESTATION

DURATION AND EVOLUTION OF LESIONS MN appear in early childhood and reach a maximum in young adulthood even though some MN may arise in adulthood. Later on, there is a gradual involution and fibrosis of lesions, but most disappear after the age of 60. In contrast, DN continue to appear throughout life and are believed not to involute (see Section 12).

SKIN SYMPTOMS MN are asymptomatic. However, MN grow and growth is often accompanied by itching. Itching per se is not a sign of malignancy, but if a lesion persistently itches or is tender, it should be followed carefully or excised, since persistent pruritus may be an early indication of malignant change.

CLASSIFICATION

MN are multiple (Fig. 9-1A) and can be classified according to their state of evolution and thus according to the histologic level of the nevus cell clusters (Fig. 9-1B).

1. Junctional melanocytic MN: These arise at the dermal–epidermal junction, on the epidermal side of the basement membrane; in other words, they are intraepidermal (Figs. 9-1B and 9-2).

2. Compound MN: Nevus cells invade the papillary dermis, and nevus cell nests are now found both intraepidermally and dermally (Figs. 9-1B and 9-3).

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