A difficult challenge that not uncommonly confronts the dermatopathologist is precise diagnosis among the various types of small cell neoplasms. Small-cell neoplasms are so named because the neoplastic cells are characteristically small, have scant cytoplasm, and show minimal differentiation. Nuclei usually appear round and hyperchromatic, resembling lymphoid cells, but they may be pleomorphic or even spindled. Thus, these neoplasms may closely resemble one another even though they are of diverse cell lines. The histologic differential diagnosis is quite broad, encompassing epithelial, lymphoid, and mesenchymal differentiation. Determining that these neoplasms are malignant is relatively easy; the challenge lies in distinguishing primary cutaneous from metastatic neoplasms and in determining cellular differentiation.
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