Derm Dx: Dark Nodule on the Shin - Dermatology Advisor

Derm Dx: Dark Nodule on the Shin

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A 37-year-old woman requests removal of a dark growth on her left leg that has increased in size over the past 2 years. She denies a family or personal history of skin cancer. The lesion is asymptomatic although she notes that shaving frequently induces irritation. Physical examination reveals a 1 cm deeply pigmented nodule on the left shin. No inguinal lymph nodes are palpable. An excisional biopsy is performed.

Deep penetrating nevus was first described over 30 years ago by Seab et al.1 The lesions were characterized by Barnhill et al as plexiform spindle cell nevi.2 The lesions are described as slightly raised and blue or darkly pigmented, most...

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Deep penetrating nevus was first described over 30 years ago by Seab et al.1 The lesions were characterized by Barnhill et al as plexiform spindle cell nevi.2 The lesions are described as slightly raised and blue or darkly pigmented, most commonly found on the head, neck, and upper trunk, making the location of the lesion in this case unusual.1.2 Deep penetrating nevi are more common in women, with the majority of lesions arising before middle age.3

Deep penetrating nevus may be difficult to differentiate clinically and histologically from melanoma. The predominant cell type of deep penetrating nevus consists of spindle cells containing granular melanin and elongated nuclei.1,2  Some cells demonstrate atypia but mitoses are uncommon. Lesions are deeply pigmented, well demarcated, and have a greater depth than width.4  

Researchers have identified mitogen-activated protein kinase pathway and β-catenin signaling activation within these lesions. These finding have led the researchers to postulate that deep penetrating nevus represent an intermediate stage in the step-wise progression from nevus to melanoma.5

Given that biopsy is necessary for diagnosis, many deep penetrating nevi will be removed in their entirety during an excisional biopsy. According to Strazzula et al, “If lesions are found to have positive surgical margins, re-excision may not be necessary given the rarity of clinical recurrence. However, any nevus with atypical features or foci concerning for malignant melanoma should be removed in its entirety with appropriate surgical margins.”3

Rebecca Geiger is a physician assistant on staff at the DermDox Center for Dermatology in Sugarloaf, PA. Stephen Schleicher, MD, is director of the DermDox Center for Dermatology, associate professor of medicine at Commonwealth Medical College, and clinical instructor of dermatology at Arcadia University and Kings College.

References

1. Barnhill RL, Mihm MC Jr, Magro CM. Plexiform spindle cell naevus: a distinctive variant of plexiform melanocytic naevus. Histopathology. 1991;18(3):243–247.

2. Seab JA Jr, Graham JH, Helwig EB. Deep penetrating nevus. Am J Surg Pathol. 1989;13(1):39–44

3. Strazzula L, Senna MM, Yasuda M, Belazarian L.  The deep penetrating nevus. J Am Acad Dermatol. 2014;71(6):1234-1240.

4. Bsirini C, Smoller BR. Histologic mimics of malignant melanoma. Singapore Med J. 2018;59(11):602-607.  

5. Yeh I, Lang UE, Durieux E, et al. Combined activation of MAP kinase pathway and β-catenin signaling cause deep penetrating nevi.Nat Commun. 2017;8(1):644.

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