A unique non-Langerhans cell histiocytosis with some features of generalized eruptive histiocytoma

A symmetric eruption of hundreds of coalescent small red macules and a few slightly elevated papules sparing the flexures was observed in a 73-year-old man. Light microscopic examination showed loose aggregates of small and large histiocytic cells. Electron microscopy showed an absence of Langerhans cell granules and lipid droplets. Features shared with generalized eruptive histiocytoma were the symmetry of the eruption sparing the flexures, the blue-red coloration, and the absence of lipid-containing foam cells and multinucleated giant cells. However, the primary occurrence of macules rather than papules or nodules, the tendency of the macules to coalesce, and the dimorphic histiocytoid infiltrate are not found in generalized eruptive histiocytoma. Nevertheless, immunohistochemistry confirmed that this unique condition is a form of MS-1+ cutaneous non-Langerhans cell histiocytosis.