Abstract

Histiocytic sarcoma is an extremely rare and aggressive neoplasm. Although it may present in numerous anatomical sites, primary cutaneous involvement is commonly reported. We report a case of a histiocytic sarcoma arising in an 80-year-old man. The patient presented with an enlarging, erythematous papule with focal ulceration on the right dorsal foot of 5 months duration. The clinical differential diagnosis included Merkel cell carcinoma and gout. Histopathologically, there was a dense dermal proliferation of atypical spindled cells with overlying ulceration. The lesional cells displayed abundant amphophilic cytoplasm with faint granularity or small vacuoles. The nuclei were irregular and markedly pleomorphic, ranging from large with pale chromatin and prominent nucleoli to small and hyperchromatic. Mitotic figures were readily identified. Immunohistochemistry revealed the lesional cells were diffusely positive for CD4 and CD163. CD68 stained cells at the periphery of the lesion, while CD31 was focally positive. Stains for cytokeratins, EMA, Melan-A, INSM1, S100 protein, CD34, and ERG were negative in tumor cells. Markers for lymphoid or myeloid lineages, including CD1a, CD3, CD20, CD45, and CD56, were restricted to sparse infiltrating immune cells. These immunohistochemical features, in the setting of a highly pleomorphic and poorly differentiated neoplasm, supported a diagnosis of histiocytic sarcoma. A subset of cases has been reported to harbor BRAF mutations. However, this lesion was negative for mutant BRAF-V600E by immunohistochemistry. Due to the rarity of histiocytic sarcoma, it may not often enter the differential diagnosis for a solitary, challenging lesion. We present this case to raise awareness of this entity in order to avoid misdiagnosis and to highlight its characteristic immunophenotype.

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