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Case Reports

Abstract

Reticulohistiocytoma is a non-Langerhans’ cell histiocytic proliferation clinically appearing as small, red-brown to yellow brown papules, often in middle aged adults. The lesions may be solitary or multiple, occurring in a variety of anatomic sites. Herein, we describe a case of reticulohistiocytoma showing aberrant expression of Melan-A. A 69-year-old man presented with a pink papule on his right lower back. Histopathologic examination revealed a dermal-based, nodular proliferation of large, but cytologically bland mononuclear cells with abundant, eosinophilic, somewhat “two-toned” cytoplasm. Scattered multinucleated forms and a minor population of admixed inflammatory cells were also noted. Immunohistochemical stains were performed for further characterization. The lesional cells were positive for CD68 and CD4, and Melan-A, while negative for cytokeratins, S100, CD1a, and CD117. The estimated Ki-67 proliferation index was low (1-2%). Given expression of Melan-A, HMB45, MIT-F, and SOX10 were added and all were negative. Melan-A was repeated and again returned positive. Despite Melan-A staining, the findings are consistent with reticulohistiocytoma. To our knowledge, Melan-A expression has not previously been described in this entity. Histiocytoid melanoma may mimic lesions of this type, however, our case lacked further immunophenotypic evidence of melanocytic differentiation. Other histiocytoses were also considered, however, Rosai-Dorfman disease generally expresses S100 and no CD1a staining was seen to suggest Langerhans’ cell histiocytosis. The absence of atypical cytology argues against histiocytic sarcoma. This case illustrates the importance of careful interpretation of immunohistochemistry in the context of a lesion’s overall light microscopic features in order to avoid misdiagnosis and overtreatment.