A 28-year-old, previously healthy woman comes to the office for evaluation of a raised, pigmented skin lesion on her leg. The patient has had no itching or pain and cannot remember how long she has had the lesion. She has used tanning machines a few times but denies excessive sun exposure. The patient is particularly concerned about the risk for malignancy because her father was recently diagnosed with skin cancer. The skin lesion is shown in the image below:
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Biopsy of the lesion reveals nests of round, uniform cells at the basal portion of the epidermis that extend into the underlying dermis. The cells contain inconspicuous nucleoli and show no mitotic activity. Which of the following is the most likely diagnosis?
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Melanocytic nevi are benign neoplasms composed of round, uniform melanocytes (nevus cells) that are mitotically quiescent. They are typically <6 mm in diameter and have a regular outline with a symmetric, sharply demarcated border and a homogeneous surface. Melanocytic nevi are progressive lesions that typically mature through the following phases:
Junctional nevi are characterized by aggregates of nevus cells limited to the dermoepidermal junction (Choice C). They typically appear as flat, black to brown macules with darker color in the center than in the periphery and preserved skin markings.
Compound nevi form as the aggregates of nevus cells extend into the dermis. They are raised papules with uniform brown to tan pigmentation. This patient's lesion is a compound nevus because it has both dermal and epidermal involvement.
Intradermal nevi are older lesions in which the epidermal nests of nevus cells have been lost. The remaining dermal nevus cells lose tyrosinase activity and produce little to no pigment. Intradermal nevi are often skin-colored, dome-shaped, and sometimes pedunculated.
(Choices A and D) Malignant melanoma is characterized by irregular and asymmetric borders with variable coloration, diameter ≥6 mm, and evolution over time. Melanomas typically expand superficially at first but may extend vertically into the dermis and subcutaneous tissues, portending a poor prognosis. Compared to nevus cells, melanoma cells are larger, contain irregular nuclei with prominent nucleoli, and have greater mitotic activity. Atypical (dysplastic) nevi can resemble melanoma clinically but typically have less extreme features. The melanocytes commonly show cellular atypia and form nests in the epidermis that fuse with nests in adjacent rete ridges.
(Choice E) Pigmented basal cell carcinoma can present as papules, nodules, or plaques with rolled borders on sun-exposed areas. They can have varying degrees of pigmentation due to functional melanocytes within the lesion. The tumor cells resemble basal epidermal cells but extend into the dermis and often form nests with a palisading arrangement (ie, parallel alignment) of peripheral cells.
(Choice F) Seborrheic keratosis forms a hyperpigmented lesion with well-circumscribed borders, a dull surface, and a "stuck-on" appearance. On microscopic examination, the lesions are composed of small cells resembling basal cells, with variable pigmentation, hyperkeratosis, and keratin-containing cysts.
Educational objective:
Compound nevi are benign proliferations of melanocytes that involve both the dermis and the epidermis. The lesions appear as slightly raised papules with uniform pigmentation and symmetric sharp borders.