Indeterminate cell histiocytosis: fact or fiction?

Indeterminate cell histiocytosis is a rare disorder, in which the predominant cells have the characteristics of both Langerhans cells and macrophages. We, in this study, describe 18 patients and compare them with those previously published. Most patients were adults with either solitary or multiple red-brown papules or nodules. While most lesions were confined to the skin, both conjunctival and bony involvement was seen. Histologically, the lesions showed patterns resembling those described for xanthogranulomas, with predominantly oncocytic (nine patients), spindled (five patients), scalloped (two patients) or vacuolated (two patients) macrophages. The accompanying infiltrate was mainly lymphocytic, although eosinophils and occasionally plasma cells were seen. All lesions were positive for macrophage markers, such as KP1 (CD68) and Ki-M1p, as well as for S-100 protein and showed variable reactivity for CD1a. No Birbeck granules were seen ultrastructurally in one patient. Some patients shared features with sinus histiocytosis with massive lymphadenopathy. It is unclear whether this disorder is a separate entity or represents various macrophage disorders identified at various time points in the inflammatory response.