Abstract

Primarily cutaneous non-Langerhans cell histiocytoses (non-LCH) are generally benign proliferations, observed in the absence of systemic symptoms or other associations. Though discrete non-LCH diagnoses were historically distinguished by unique clinical and histopathologic findings, a limited number of reports suggest that previously presumed pathognomonic parameters are in fact not mutually exclusive and that these entities instead represent a continuous spectrum with overlapping features. In the current case, a 49-year-old woman with systemic lupus presented with an enlarging tender bump on her lip that failed to improve with oral steroids or antibiotics, prompting excision for symptomatic relief and definitive diagnosis. Histopathologic analysis presented a diagnostic dilemma due to a discrepancy between findings on routine sections and the expected immunohistochemical staining patterns. A well circumscribed dermal nodule contained neutrophils and lymphocytes intercalating between large histiocytes, many with ground-glass two-toned cytoplasm suggestive of a reticulohistiocytoma. However, other large cells within the nodule contained bubbly cytoplasm reminiscent of Touton giant cells within a xanthogranuloma. In addition, emperipolesis was readily observed and S100 staining diffusely positive throughout the nodule, both hallmark findings of cutaneous Rosai-Dorfman disease (RDD). Ultimately, taking together the available clinical and histopathologic findings, a diagnosis of reticulohistiocytoma was rendered. The current case adds to the existing literature by highlighting unexpected emperipolesis and S100 expression in a pleomorphic histiocytic proliferation and invites additional investigation to definitively establish the frequency of these findings in non-LCH that do not otherwise fit criteria for RDD.

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