Deep penetrating nevus (DPN), also known as a plexiform spindle cell nevus, is a darkly pigmented melanocytic lesion that most often appears as an asymptomatic solitary papule or nodule on the head or neck, trunk, or upper extremities. The DPN is often described by patients as a new or changing lesion, since it is almost always acquired during life. It usually arises before age 30, and less than 5% of DPN develop after the age of 50. DPN are more common in females than males, and there is no association with a family history of melanoma.
Clinically and histopathologically, DPN can be confused with malignant melanoma since it can arise with sudden onset, have a darkly pigmented appearance, and have cytologic pleomorphism with deep infiltration of the dermis. Dermatopathologists thus face the primary challenge of differentiating between DPN and malignant melanoma, as well as other melanocytic lesions.
DPN are generally considered benign, although rare cases of recurrences have been noted as well as spread to regional lymph nodes, mostly in the setting of having atypical histopathologic features.
ICD10CM: D22.9 – Melanocytic nevi, unspecified
SNOMEDCT: 402549006 – Deep penetrating melanocytic nevus
Differential Diagnosis & Pitfalls
Cellular blue nevus – Presents as a blue-gray nodule on the scalp or trunk that is <3 cm in diameter; histopathology reveals a proliferation of pigmented spindled and dendritic melanocytes that can be biphasic and rarely have cytologic atypia or mitoses. Nests of melanocytes at the dermoepidermal junction are typically not seen.
Spitz nevus – Presents as a pink to brown papule or nodule that is <1 cm in diameter, often on the face; histopathology reveals nests of large epithelioid or spindled cells that mature on extension into the dermis and rarely contain mitoses. Hyperkeratosis, hyperplasia, and hypergranulation in the epidermis can be seen.
Pigmented spindle cell nevus of Reed – Presents as a brown-to-black macule or papule that is <6 mm in diameter with a starburst pattern on dermoscopy. Histopathology often reveals vertically oriented nests of heavily pigmented, spindled melanocytes with overall symmetry. Low-grade cytologic atypia may be seen, but mitoses are rare.
Melanoma – Clinically and histopathologically, melanoma may have varied presentations; however, significant asymmetry, multiple colors, border irregularity, and >1 cm in size in an older patient are features not typically seen in DPN. On histopathology, an asymmetric infiltrative melanocytic proliferation with severe cytologic atypia and frequent mitoses should raise concerns for melanoma. An intraepidermal or in situ component can be present in certain melanoma subtypes unlike in DPN.