This is a rare, but highly aggressive tumor which has a propensity for damaged skin. It usually affects the head and neck or the extremities. There is a predilection for caucasians and males. A wide age range may be involved, but the vast majority are over 50 years old. The tumor presents as a firm raised painless nodule that usually measures 2 cm or less and slowly increased in size. The overlying skin may be erythematous, violaceous or purple.
This is a neuroendocrine carcinoma of the skin and its precise histogenesis is unknown although immunohistochemical profile suggests it is derived from epidermal Merkel cells. Histologically the tumor is in the dermis and often extends into the subcutaneous fat. There is a variety of histological subtype including trabecular, intermediate, and small cell variant. The intermediate type is the most common and is composed of nodules of diffuse sheets of basophilic tumor cells with sparse cytoplasm and vesicular nuclei containing small nucleoli. Dissection of collagen or fat, nuclear moulding, and possible foci of necrosis or widespread apoptosis are often seen as well.
The tumor can be confused for basal cell carcinoma, lymphomatous deposits, small cell melanoma, and metastatic small cell carcinoma. Immunohistochemicaly, these tumors are positive for CK 20 (often evident as a paranuclear globule), NSE, chromogranin A, synaptophysin, CD 56, CD57, and EMA. They are also frequently positive for BCL-2, FLI-1, CD99, and MAP-2. Typically these tumors are negative for CK 7, MASH-1, S100, and vimentin, and TTF-1.