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Pathology of Merkel Cell Carcinoma  

Dr Sampurna Roy MD         






The tumour cells express cytokeratin 20 which is characteristic of Merkel Cell Carcinoma. 

Merkel cell carcinoma is a rare, neuroendocrine cancer of skin.

This tumour was originally described under the name of "trabecular carcinoma" by Toker in 1972.

Merkel cell carcinoma is usually found in the sun-exposed areas of head, neck and extremities and rarely as a metastatic disease of unknown primary, usually in lymph nodes.

Clinically, the tumour presents as a solid, non-ulcerated, painless, reddish violet nodule or firm plaque which is usually less than 2cm in diameter.

Histologically, the tumour cells are usually located in the dermis.

Purely intraepidermal in-situ Merkel cell carcinoma has also been reported. 

Architectural patterns range from trabecular to insular or diffuse type.

Often the intradermal tumour fills the entire dermis and is separated from the epidermis by thin Grenz zone.

The tumour may be associated with Bowen's disease, squamous cell carcinoma and basal cell carcinoma.

Glandular differentiation may be present.
Cytologically the tumour cells are monomorphic with scanty amphophilic to eosinophilic cytoplasm.

Nuclei are uniform and some cells exhibit nuclear moulding.

The chromatin pattern is finely granular.

Small nucleoli may be present. Mitotic figures and apoptotic bodies are often numerous. 

Histological variants include desmoplastic, epidermotropic and pagetoid types.

Immunohistochemistry plays a major role in establishing the diagnosis.

The tumour cells express cytokeratin 20 in a dot-like paranuclear pattern with perinuclear crescentic pattern.

Other low molecular weight cytokeratin antibodies like CAM5.2, MNF116 also show similar patterns.

Neurofilament is expressed in the cytoplasm of most Merkel cell carcinoma.

CK20 and neurofilament immunohistochemistry support the diagnosis of primary Merkel cell carcinoma of skin and rules out metastatic small cell carcinoma, most commonly from a lung primary.


The main differential diagnosis of Merkel cell carcinoma are PNET, lymphoma, small cell melanoma, metastatic small cell carcinoma and squamous cell carcinoma.

Appropriate antibody panel may be helpful in the differential diagnosis of cutaneous  "small round blue cell" tumour.

The panel includes Cytokeratin (CK), Cytokeratin 20, Neurofilament, S-100 protein, Epithelial membrane antigen (EMA) , carcinoembryonic antigen (CEA) ,Neuron-specific enolase NSE, CD99 and LCA.


Further reading

Neuroendocrine differentiation in primary Merkel cell carcinoma--possible prognostic significance.

Merkel cell carcinoma in situ.

B-Cell Specific Activation Protein Encoded by the PAX-5 Gene Is Commonly Expressed in Merkel Cell Carcinoma and Small Cell Carcinomas.

Cyclooxygenase-2 expression in primary Merkel cell carcinoma.

CD117, CK20, TTF-1, and DNA topoisomerase II- antigen expression in small cell tumors.

Immunostaining for cytokeratin 20 improves detection of micrometastatic Merkel cell carcinoma in sentinel lymph nodes.  

CD99 and cytokeratin-20 in small-cell and basaloid tumors of the skin.

Pagetoid intraepidermal spread in Merkel cell (primary neuroendocrine) carcinoma of the skin.




Dr Sampurna Roy  MD

Consultant  Histopathologist (Kolkata - India)






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