Histiocytic sarcoma with interdigitating dendritic cell differentiation: a case report with fine needle aspiration cytology and review of literature.

A hybrid histiocytic sarcoma-interdigitating dendritic cell sarcoma was found in a small perinephric lymph node of an asymptomatic 80-year-old man, who presented a year ago with two small foci of lung metastasis found during routine chest X-ray. Fine needle aspiration cytology demonstrated interconnecting long and thin cell processes radiating from dendrite-like neoplastic cells with oval, enlongated, reniform, and irregular nuclei with vesicular chromatin and distinct nucleoli. Histology showed spindled epitheliod and histiocytic cells with abundant, slightly eosinophilic cytoplasm with indistinct cell borders and forming fascicles in a vague storiform pattern with interspersed T-lymphocytes. Immunohistochemically, the neoplastic cells were strongly positive for histiocytic markers: CD163, CD68, lysozyme, and PU.1, as well as strongly positive for dendritic cell markers: S100 and fascin, but were negative for CD1a (Langerhans cell marker), CD21/CD35 (follicular dendritic cell markers), B-cell, and T cell markers. This case is compared to the four hybrid histiocytic-dendritic sarcomas reported since 1983.