Letter to Editor: Characterizing the Lack of Specificity of the Histopathological Findings Seen in Multicentric Reticulohistiocytosis and Reticulohistiocytoma.

Both reticulohistiocytoma and multicentric reticulohistiocytosis are defined by their shared histopathologic appearance of dermal proliferations of mononuclear histiocytes and multinucleated giant cells with eosinophilic "ground-glass" or "two-toned" cytoplasm. To assess the specificity of the histopathologic findings seen in reticulohistiocytoma and reticulohistiocytosis, this study retrospectively examined 109 random, unscreened excision specimens of ruptured cysts to determine at what frequency "ground-glass" or "two-toned" histiocytes and multinucleated giant cells are seen in reactive cutaneous infiltrates. Slightly more than half (56.9%) of the ruptured cyst cases had "ground-glass" histiocytes. The majority (84%) of the ruptured cyst cases had multinucleated giant cells. The shared histologic findings associated with reticulohistiocytoma and reticulohistiocytosis are much more pervasive than previously documented and are relatively non-specific. Identification of immunohistochemical and/or molecular biomarkers that distinguish reticulohistiocytoma and reticulohistiocytosis from histologic mimickers would likely prove indispensable.