Histiocytic sarcoma: identification of its histiocytic origin using immunohistochemistry.

We describe a 56-year-old woman with histiocytic sarcoma involving the bone marrow. The neoplastic cells proliferated diffusely and showed occasional erythrophagocytosis. Immunohistochemically, the neoplastic cells were positive for CD68, lysozyme, CD4 and CD163, but negative for B- and T-cell markers, S100 protein and epithelial markers. The patient received multi-agent chemotherapy and is living at 22 months after diagnosis without recurrence. Histiocytic sarcoma is an exceedingly rare hematopoietic neoplasm and the prognosis is poor due to its rapid progression, widespread disease and poor response to therapy. It is important to recognize this rare neoplasm and to confirm the diagnosis using specific immunohistochemical markers.