Answer: Merkel cell carcinoma

Histology: The tumor is centered in the dermis, extends into the subcutaneous tissue and has no direct connection to the overlying epidermis. The tumor is densely cellular and the cells grow in a sheet-like pattern. The way the cells percolate through the fat without inducing much of a stromal reaction is somewhat reminiscent of lymphoma. The tumor cells are small, round, and blue. They have very scant cytoplasm and a round nucleus with evenly dispersed chromatin giving them a characteristic “smoky” appearance. Nucleoli are not prominent. Mitotic activity is very high.

Discussion: Merkel cell carcinoma is a high-grade neuroendocrine carcinoma of the skin. It most commonly arises in the face and extremities of older patients. Like its counterpart in the lung, it is an aggressive tumor with a propensity to metastasize regionally and distally. At the morphologic level, it can be very difficult to distinguish from other poorly differentiated tumors with small-round cell morphologic. In particular, it may be very difficult to morphologically distinguish a primary cutaneous neuroendocrine carcinoma from a metastatic neuroendocrine carcinoma (e.g. pulmonary small cell carcinoma). In addition to clinical evaluation to rule out a lung primary, recent advances in the immunohistochemical armament have increasingly facilitated this distinction. Immunohistochemical markers that are consistently expressed in Merkel cell carcinoma but not metastatic small cell carcinoma include Cytokeratin 20 (often perinuclear dot-like pattern) and neurofilament. Conversely, the thyroid transcription factor, TTF-1, is expressed in most small cell lung cancers but not in Merkel cell carcinomas.