Benign Melanocytic Neoplasms

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92

Benign Melanocytic Neoplasms

Benign Pigmented Cutaneous Lesions Other Than Melanocytic Nevi

• This group of lesions can further be divided into: (1) predominantly epidermal lesions (Table 92.1; Figs. 92.1–92.5); and (2) dermal melanocytoses (Table 92.2; Figs. 92.6 and 92.7).

• In the predominantly epidermal lesions, the tan to brown color can result from a variety of mechanisms – e.g. increased melanocyte activity (melanogenesis), increased melanin content in keratinocytes, and a mild increase in the number of melanocytes.

• In dermal melanocytoses, the skin is blue to blue-gray in color (ceruloderma) due to the presence of melanin-producing melanocytes in the mid to lower dermis and the resultant Tyndall phenomenon (the preferential scattering of shorter wavelengths of light by the dermal melanin).

Acquired Melanocytic Nevi (Moles)

• Benign proliferations of a type of melanocyte called a ‘nevus cell’.

• Nevus cells differ from ‘ordinary’ melanocytes, which typically reside as single units in the basal layer of the epidermis, in that they: (1) usually cluster as nests in the lower epidermis and/or dermis; and (2) do not have dendritic processes (except when found in a blue nevus).

• Both ‘ordinary’ melanocytes and nevus cells can produce melanin.

• Acquired melanocytic nevi can be categorized as common (banal) or atypical (dysplastic), and they are further named based on the histologic location of the collections of nevus cells (Fig. 92.8):

– Junctional melanocytic nevus: dermal-epidermal junction.

– Compound melanocytic nevus: dermal–epidermal junction plus dermis.

– Intradermal melanocytic nevus: dermis.

• Variants include halo, blue, Spitz, and ‘special site’ nevi (Fig. 92.9).

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Fig. 92.9 Variants of acquired melanocytic nevi. A The three most common types of acquired melanocytic nevi. A1 Junctional nevus. Clinically, a brown macule with central hyperpigmentation. Dermoscopically, a uniform pigment network. A2 Compound nevus. Light to medium brown papule. A3 Intradermal nevus. A light tan, soft, raised papule. Dermoscopically, focal globular-like structures, whitish structureless areas, and fine comma vessels. B Clinically atypical acquired melanocytic nevi. B1 Multiple pigmented macules and papules of varying sizes on the back. B2 Close-up photo; the dermoscopy pattern is reticular-disorganized and can be seen with uncertain lesions. B3 ‘Fried egg’ appearance, with a central elevated soft papule and macular rim. C Blue nevi. C1 Common blue nevus. By dermoscopy, blue homogeneous color typically found in blue nevi. C2 Cellular blue nevus. A firm blue plaque is a common presentation. D Spitz nevi. D1 Classic Spitz nevus. Red dome-shaped papule on the ear of a child. D2 Reed nevus, typified dermoscopically by the classic starburst pattern (regular streaks at the periphery of a heavily pigmented and symmetric small macule). E Nevi of ‘special sites’ (e.g. scalp, acral, genital, milk-line). E1 Acral nevus. A brown macule on the sole of the foot. Dermoscopically, a lattice-like pattern is seen. F Other ‘specially named’ nevi. F1 Eclipse nevus. A tan center and thinner brown rim; note the stellate appearance of the brown rim. F2 Cockade or target nevus. Central lightly pigmented papule surrounded by a tan annulus then a brown ring. F3 One variant of combined melanocytic nevus. Dark brown to black papule within an otherwise uniformly pigmented light brown nevus. The differential diagnosis includes the possibility of a melanoma developing in a nevus. F4 Recurrent nevus. Dark brown pigmentation within the center of a circular scar; the pigmentation reflects the proliferation of melanocytes within the epidermis. Courtesy, Giuseppe Argenziano, MD, Raymond L. Barnhill, MD, Jean L. Bolognia, MD, Lorenzo Cerroni, MD, Harold S. Rabinovitz, MD, Ronald P. Rapini, MD, and Iris Zalaudek, MD.

• Risk factors for developing acquired melanocytic nevi: (1) a family history of numerous nevi; (2) a greater degree of sun exposure during childhood, especially intermittent and intense; and (3) lightly pigmented skin (individuals with phototype II have the greatest number of nevi).

• The vast majority of acquired melanocytic nevi remain as benign neoplasms throughout one’s life and do not require treatment.

• Having >100 melanocytic nevi (8- to 10-fold increased relative risk) or multiple atypical nevi (>5 = 4- to 6-fold increased relative risk) are phenotypic markers for an entire skin surface at risk for developing cutaneous melanoma; such persons should have lifelong surveillance with periodic total body skin examinations (beginning around puberty) and counseling regarding home self-skin examinations and sun protective measures.

• Cutaneous melanoma may arise within a pre-existing nevus, but more than half of cutaneous melanomas arise de novo – i.e. in previously normal-appearing skin.

• Most patients with numerous nevi and atypical nevi will have a prominent morphologic type of nevus (‘signature nevus’); by recognizing signature nevi, the ‘ugly duckling’ can be identified and closely examined.

• Persons with more darkly pigmented skin will typically have darker colored nevi.

• Rx: it is not necessary to remove clinically atypical nevi prophylactically in order to confirm the presence of histological atypia; biopsies of nevi are indicated primarily when a severely atypical nevus or cutaneous melanoma is in the DDx (Table 92.4); if banal nevi become irritated (e.g. by clothing or jewelry), shave removal can be done.

• When cutaneous melanoma is in the DDx, complete removal of the nevus is recommended as well as submission to a dermatopathologist; removal can be accomplished by several methods (see Fig 1.6); providing additional information (e.g. eccentric hyperpigmented area) is also helpful.

Atypical (Dysplastic or Clark’s) Acquired Melanocytic Nevi

• Benign lesions that share, to a lesser extent, many clinical features of cutaneous melanoma (e.g. asymmetry, border irregularity, color variegation, and diameter >6 mm) but usually they ‘age’ in a manner similar to banal nevi and only a tiny proportion may develop melanoma within them.

• Histologically, architectural disorder is seen and cytologic atypia may be present; sometimes the latter is categorized as mild, moderate, or severe.

• The major risk factor for developing atypical nevi is genetic predisposition.

• The ‘atypical mole syndrome’ has been variably described in the literature, ranging from individuals with atypical nevi but no personal or family history of melanoma to the familial atypical multiple mole and melanoma syndrome (FAMMM syndrome), in which an individual with numerous nevi and ≥2 first-degree relatives with cutaneous melanoma has a very high risk of developing melanoma.

• Atypical nevi often appear around puberty and are thought to develop throughout life.

• Although the majority of atypical nevi are located in intermittently sun-exposed sites (e.g. trunk and extremities [lower > upper in females]), often they can be seen on the breasts and buttocks.

• Clinically, atypical nevi are recognized by various features – e.g. varied colors (pink, brown, tan), a ‘fried-egg’ appearance with a papular component and a macular rim, a larger size than banal nevi, and borders that are ill-defined, ‘fuzzy,’ and sometimes notched or irregular (see Fig. 92.9).

• Adjuncts to monitoring these patients include baseline and follow-up nevus and total body photography; dermoscopy; patient/partner education on skin self-examination and signs of melanoma; and possibly alternating examinations between two dermatologists.

• Nevi with mild cytologic atypia histo­logically and most with moderate cytologic atypia do not require re-excision, especially when the biopsy attempted to remove the entire nevus; it is generally recommended, however, that severely atypical nevi be completely excised or re-excised with conservative margins (e.g. 3–5 mm).

Halo Nevus

• A melanocytic nevus that is surrounded by a round or oval, usually symmetric, halo of complete depigmentation (i.e. white color).

• The central nevus is most often a common acquired melanocytic nevus, but it can also be other nevus subtypes.

• The halo of depigmentation is believed to represent a T-cell-mediated immune response against nevus antigens, analogous to vitiligo (see Chapter 54).

• Halo nevi are seen in up to 5% of Caucasian children aged 6–15 years; they are more common in patients with an increased number of nevi and a personal or family history of vitiligo.

• The most common location is the back; multiple halo nevi are seen in ~50% of cases.

• There are four clinical stages in the life of a halo nevus, and the evolution usually occurs over years to decades (Fig. 92.10).

• The central nevus should be assessed for suspicious or clinically atypical features; if none are present, then no treatment is necessary (vast majority of lesions); if present, then biopsy of just the central nevus should be performed.

• A new onset of multiple halo nevi is unusual in middle-aged and older adults; this clinical scenario should raise the possibility of the halo nevi representing an autoimmune reaction against a cutaneous or ocular melanoma.

• DDx: other pigmented lesions that can have halos (e.g. solar lentigo, seborrheic keratosis), halo primary cutaneous melanoma, halo melanoma metastasis.

Blue Nevi

• Benign lesions due to a proliferation of dendritic melanocytes within the dermis; blue in color because of the Tyndall effect (see above).

• Sites of predilection are those areas in which active dermal melanocytes are still present at the time of birth (e.g. head, neck, dorsal hands and feet, sacral region).

• Most blue nevi have a somatic activating mutation in GNAQ, which encodes the Q-class of G-protein α subunits.

• The common blue nevus typically presents as a solitary, blue to blue-black, dome-shaped papule, usually <1 cm in diameter and most often occurring on the dorsal hands or feet (see Fig. 92.9); frequently arises in adolescence; no malignant potential.

• The cellular blue nevus tends to be larger in size (i.e. more plaque-like), and it can be congenital or acquired (see Fig. 92.9); favors the head, sacral region, and buttocks; melanoma (malignant blue nevus) can develop within cellular blue nevi (see Chapter 93); if location (e.g. scalp) or patient awareness prevents satisfactory observation, then complete excision can be performed.

• The combined blue nevus typically presents as a blue to gray-brown macule or papule with a subtle brown rim; it usually represents a combination of a blue nevus with a banal nevus, but in theory can be a combination of any two types of nevi.

• DDx: traumatic tattoo (e.g. carbon from a pencil), which typically can be distinguished by history, or a venous lake (compressible) > melanoma metastasis, primary cutaneous melanoma.

• Rx: none for common blue nevi and as noted above for cellular blue nevi; lesions with a history of recent growth or change should be biopsied.

• Multiple blue nevi (normal in Asians) may signify an underlying syndrome, e.g. Carney complex (see Table 92.3).

Congenital Melanocytic Nevi (CMN)

• Classically defined as a melanocytic nevus present at birth; may be subtle at birth and not readily apparent for a few months.

• Melanocytic nevi that become apparent after 3 months of age, but before 2 years of age, have been termed ‘tardive CMN,’ ‘congenital nevus-like nevi,’ or ‘early onset nevi’.

• CMN are due to a proliferation of melanocytes that arise during embryogenesis; melanocytes extend deep into the dermis and subcutaneous tissues and often follow follicular and neurovascular structures; NRAS mutations have been detected in CMN and associated neurologic lesions (see below).

• Incidence of small CMN is estimated at 1–2%, with large CMN having an incidence of ~1 in 20 000 individuals.

• CMN are classified into four groups, based on their final adult size:

– Small CMN: <1.5 cm (Fig. 92.11A).

– Medium CMN: 1.5–20 cm (Fig. 92.11B–D).

– Large CMN: >20–40 cm (in a neonate, large CMN are >9 cm on the head or >6 cm on the body).

– Giant CMN: >40 cm (Fig. 92.11E).

• Small or medium-sized CMN are usually solitary, can occur anywhere on the body, range in color from tan to black, and may have increased terminal hair growth (hypertrichosis).

• Giant CMN are sometimes referred to as ‘bathing suit’ or ‘garment’ nevi because of their distribution pattern; multiple smaller, widely disseminated ‘satellite’ nevi are also commonly associated with these giant CMN.

• CMN grow proportionately with the child and over time can change from initially flat, evenly pigmented patches to elevated, mottled, pebbly or verrucous plaques; scalp CMN may lighten in color and gradually regress; papules or nodules can develop within the CMN, but signs of rapid growth, induration, or ulceration should raise suspicion for the development of melanoma and prompt a biopsy.

• CMN are a known risk factor for melanoma and neurocutaneous melanosis (NCM), with the absolute risk being associated with the severity of the cutaneous phenotype.

• The risk of developing melanoma within a small or medium-sized CMN is thought to be <1% over a lifetime; the risk for giant CMN is ~5%, with greater risk being a function of nevus size and number of satellite lesions; in giant CMN, the majority of melanomas develop in childhood (~50% in the first 5 years of life).

• Cutaneous melanomas can arise within the dermis or subcutaneous tissues of a CMN, making clinical diagnosis difficult.

• Patients with a giant CMN in a posterior axial location that is associated with multiple satellite nevi (≥20) have the greatest risk of developing melanoma; although the melanoma can develop within the CMN itself (not the satellite lesions), the CNS, or the retroperitoneum, the primary site may remain unknown.

• In neurocutaneous melanosis (NCM), also referred to as leptomeningeal melanocytosis, there is a proliferation of melanocytes in the CNS (as well as the skin); individuals with multiple (≥3, but usually 10–20+), disseminated medium-sized CMN are at greatest risk of NCM, followed by those with a giant CMN in a posterior axial location with multiple satellites.

• NCM-associated signs – e.g. hydrocephalus, seizures, developmental delay, increased intracranial pressure, cranial nerve palsies, sensorimotor defects, and hypotonia – tend to manifest by 2–3 years of age.

• An approach to the management of CMN is presented in Table 92.5 and Fig. 92.12.

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