Merkel Cell Carcinoma — SCE Dermatology MCQ

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ModerateSkin CancerMerkel Cell CarcinomaSCE Dermatology

A 60-year-old man has a rapidly growing nodular tumour on his scalp. Biopsy shows small blue round cells with scant cytoplasm. Immunohistochemistry is positive for CK20 (perinuclear dot pattern), synaptophysin, and chromogranin. It is negative for S100, CK7, and TTF-1. What is the diagnosis?

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Correct answer: C — Merkel cell carcinoma

The immunohistochemical profile of CK20 positive (perinuclear dot pattern — highly specific), synaptophysin/chromogranin positive (neuroendocrine markers), with negative S100 (excludes melanoma), negative CK7 (helps exclude small cell lung carcinoma which is CK7+/CK20-), and negative TTF-1 (excludes pulmonary origin) is diagnostic of Merkel cell carcinoma. MCC is an aggressive neuroendocrine cutaneous carcinoma with high metastatic potential. The CK20 perinuclear dot pattern is the single most useful immunohistochemical marker. Approximately 80% of MCC is associated with Merkel cell polyomavirus. Management requires wide excision, SLNB, adjuvant radiotherapy, and for advanced disease, checkpoint inhibitor immunotherapy (Avelumab).

Reference: BAD 2021 MCC; NICE