Merkel Cell Carcinoma
Clinical
An aggressive neoplasm typically presenting as a
slowly growing nodule on the sun exposed skin (head
and neck region) of older adults
The recurrence and metastatic rate is approximately
40-50%
Microscopic
A variety of histologic forms may be seen and include
sheet-like, ribboned, nested, trabecular and organoid
variants
Pseudorosettes may be prominent
These tumors are typically dermal based but frequently
involve the subcutis and may show an intraepidermal
growth pattern
Focal areas of squamous, eccrine or sebaceous differ-
entiation may be seen, and these tumors may arise in
conjunction with another, histologically distinct
neoplasm
The cytologic and immunophenotypic appearance is
characteristic and common to all variants
Cytologically, the tumor cells have very high nuclear/
cytoplasmic ratios, indistinct cell borders, hyperchro-
matic, and finely granular nuclei with inconspicuous
nucleoli and thin nuclear membranes
Nuclear molding and mitoses are abundant
Immunophenotypically, the tumor cells express low
molecular weight cytokeratin (CAM 5.2) in a peri-
nuclear dot-like pattern which may also be seen with
neurofilament staining
Cytokeratin 20 staining is generally positive, and these
tumors express a variety of neuroendocrine markers
including neuron-specific enolase, chromogranin,
synaptophysin and neurofilament, but are generally
negative for S100 and vimentin