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 MCC 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.
Immunocytochemistry plays a major role in establishing the diagnosis.
The tumour cells express cytokeratin20 in a dot-like paranuclear pattern
with perinuclear crescentic pattern.
Other low molecular weight cytokeratin antibodies eg CAM5.2,MNF116 also
show similar patterns.
Neurofilament is expressed in the cytoplasm of most MCC.
CK20 and neurofilament immunohistochemistry support the diagnosis of
primary MCC of skin and rules out metastatic small cell carcinoma, most
commonly from a lung primary.
The main differential diagnosis of MCC 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 CK, CK20, NF,
S100, EMA, CEA, NSE, CD99 and LCA.
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