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MERKEL CELL CARCINOMA (PRIMARY CUTANEOUS NEUROENDOCRINE CARCINOMA)

Merkel cell carcinoma (MCC) is an uncommon but often aggressive tumor, with a predilection for the head and scalp in elderly patients (Figs 33.54 and Figs 33.55). Immunosuppression is a major risk factor.
    Merkel cell carcinomas usually present as rather vascular and rapidly enlarging nodule(s). Lymph node metastasis has occurred in about 20% of patients at the time of presentation.
    The clinical differential diagnosis is usually from other rapidly growing neoplasms, especially squamous cell carcinoma, amelanotic melanoma, cutaneous metastasis, or benign lesions such as pyogenic granuloma. Histologically, the differential diagnosis can be very difficult; it includes other small cell carcinomas such as metastatic small cell lung cancer, and even basal cell carcinoma may be difficultto exclude on routine histopathology. Staining with neuron-specific enolase or cytokeratins may show a characteristic dot-positivity; recently, cytokeratin 20 immunoreactivity has been shown to be a useful marker to distinguish MCC from other small cell carcinomas, in which positivity is rare, and patchy when it does occur (by contrast, cytokeratin 7 is positive in small cell lung cancer but is negative in MCC).
    Treatment is traditionally with wide local excision, radiotherapy, and chemotherapy in more aggressive cases. Experience with micrographic surgery, sentinel node biopsy, and monoclonal antibodies such as tumor necrosis factor may influence this management in the future.

PRACTICE POINTS

White/Cox: Diseases of the Skin, 2ed.(c) 2006, Elsevier Inc. All rights reserved.