Merkel cell carcinoma
- Lesion
A rare, aggressive skin cancer of neuroendocrine cells, often caused by a virus. It looks like a fast-growing red bump on older or immunosuppressed people.
What it is
Merkel cell carcinoma is a primary neuroendocrine carcinoma of the skin, a small round blue cell tumour that grows rapidly and spreads early to lymph nodes. Most cases in the northern hemisphere are caused by clonal integration of Merkel cell polyomavirus, whose large T antigen inactivates RB; the rest are driven by UV with a very high mutation load. It is more common in older people and in immunosuppression.
Structure
- Dermal nodule of sheets and trabeculae of small cells, often sparing the epidermis.
- Nuclear moulding, crush artefact and apoptotic bodies.
- Electron microscopy: dense-core neurosecretory granules.
Markers
- CK20 (perinuclear dot)
- Synaptophysin
- Chromogranin
- INSM1
- Neurofilament
- TTF1 negative
Clinical notes
- Differential of small blue cells in skin: Merkel cell carcinoma, metastatic small cell lung carcinoma, lymphoma, melanoma; immunostains decide.
- Immunosuppression (transplant, HIV, chronic lymphocytic leukaemia) increases risk.
Test yourself
Think of the answer, then tap the question to see it.
Which virus is associated with Merkel cell carcinoma?
Merkel cell polyomavirus, whose integrated large T antigen inactivates RB.
How do you distinguish Merkel cell carcinoma from metastatic small cell lung carcinoma?
Merkel is CK20-positive in a perinuclear dot and usually TTF1-negative; small cell lung carcinoma is CK20-negative and TTF1-positive.
Which immunotherapy class is effective in Merkel cell carcinoma?
Immune checkpoint inhibitors (PD-1 or PD-L1 blockade).
Elsewhere in DurgiPedia
- Keratinocytes, melanocytes, Langerhans and Merkel cells
- Oncogenic viruses and H. pylori: the table to know
- Immunohistochemistry: the principle and the markers to know
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Merkel Cell Carcinoma Treatment (PDQ®)–Health Professional Version (National Cancer Institute, 2025).
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Representative model, for educational purposes only.