Cutaneous indeterminate cell histiocytosis: a new spindle cell variant resembling dendritic cell sarcoma

Background: Cutaneous indeterminate cell histiocytosis is a rare neoplastic disorder. Its varied histological presentation and rarity have limited efforts to determine its pathogenic relationship with other histiocytic lesions and possibly, its recognition. Methods: We report on an unusual histologic pattern of indeterminate cell histiocytosis that resembled follicular dendritic sarcoma. A battery of immunohistochemical stains and electron microscopy were performed to elucidate the phenotype of the “histiocytic” cells. Based on a review of the literature, reported cases of indeterminate cell histiocytosis are presented and the diagnostic differential of spindle‐cell lesions is discussed. Results: Spindling histiocytes were positive for S‐100 and CD1a. The monocytic/macrophage marker, CD68, and the dendritic cell marker, CD21, were negative. Electron microscopy failed to reveal Birbeck granules. Conclusions: Relatively few reports of indeterminate cell histiocytosis exist, some of which include discussion of potential overlaps with the non‐X histiocytoses. Although the presence of prominent spindling in our case expanded the differential to include non‐histiocytic disorders, the identified histiocytes unequivocally fulfilled the criteria of S‐100 and CD1a positivity without demonstrable Birbeck granules.