Pathology of Merkel Cell Carcinoma  

Dr Sampurna Roy MD          2022

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

Merkel cell carcinoma (MCC) 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.

Polyomavirus (MCPyV) is associated with Merkel cell carcinoma (MCC).

Merkel cell Polyomavirus (MCPyV) was identified in January 2008 by Feng and colleagues in tumour tissue from MCC patients, proving clonal integration of the virus DNA into the host genome . Meanwhile several studies confirmed this observation showing frequent prevalence of MCPyV DNA in MCCs. These data suggest MCPyV as the likely causative agent of MCC.

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.

Dermatopathology Quiz Case: 147

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.