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Deep Penetrating Nevi
The so-called deep penetrating nevus (DPN) is a variant of a benign melanocytic nevus. It is composed of distinctive pigmented spindled, ovoid, or occasionally epithelioid melanocytes with a characteristically inverted triangle-like downward architecture. Its clinical significance lies in the potential confusion with melanoma.
Clinical Findings
The clinical appearance of a DPN often resembles that of a blue nevus. The lesions usually present as circumscribed, dark brown, or bluish black macule or papule ( Fig. 6.1 ). Most DPN are small (<6 mm in diameter). They occur in patients of all ages, but most of them are seen and biopsied in children and young adults. Most lesions are thought to be acquired, coming to clinical attention usually after puberty, but some may be congenital (tardive congenital nevus, with pigmentation and clinical visualization developing years after the initial formation of the lesion). No predilection for a specific anatomic region has been established. Deep penetrating nevi commonly develop as a change in a preexisting nevus, forming a combined nevus.
Histopathologic Findings
Deep penetrating nevi are usually well-circumscribed dermal-based melanocyte proliferations composed of pigmented spindle, ovoid, and epithelioid cells, often displaying a deep wedge-shaped (inverted triangle-like) silhouette at scanning magnification ( Figs. 6.2–6.4 ; Box 6.1 ). The melanocytes are typically arranged in fascicles and commonly display a plexiform growth pattern. The lesion often extends to the deep reticular dermis and/or subcutis. However, small superficial variants exist, in which the tumor cells are confined to the upper and mid-reticular dermis; the characteristic cytologic and architectural features allow recognition of the tumor ( Fig. 6.5 ). Fascicles of plump spindle cells and nested aggregates of ovoid or epithelioid cells are commonly surrounded by melanophages. The extent of melanization is usually relatively uniform throughout the lesion, but may vary from light to more darkly pigmented. Tumor cells tend to grow along neurovascular and adnexal structures (see Figs. 6.2 and 6.5–6.7 ). They may be present in the pilar erector muscle. In some, especially larger lesions, the melanocytes may form expansile aggregates with bulbous contours at the base of the tumor. Smaller lesions may lack a bulbous component and have a broad-based dome-shaped silhouette, or display small sausage-like extension at the base following neurovascular and/or adnexal structures (see Figs. 6.5 and 6.6 ).
Box 6.1
Deep Penetrating Nevus
Clinical Findings
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Brown or bluish-black macule or papule
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Usually presents in children and young adults, but occurs at any age
Histopathologic Findings
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Symmetric silhouette of an inverted triangle (wedge-shaped)
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Often extends into deep dermis/subcutis
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Occasionally confined to the superficial or mid-dermis only
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Plexiform growth pattern common
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Usually pigmented
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Range of cytologic features with variable proportions and shapes of spindle, ovoid, and epithelioid melanocytes
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Cytoplasm often contains fine melanin granules
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Focal nuclear atypia common (enlarged nuclei, hyperchromatism, pseudoinclusions)
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Curvilinear spindle cells around groups of melanocytes
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Mitoses are absent or rare
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Associated melanophages common
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