A malignant tumor arising from interdigitating cells; light microscopical, ultrastructural, immuno-and enzyme-histochemical characteristics.

A tumor in a 37 years old male is described in which the tumor cells appeared to be derived from interdigitating cells normally found in the T-cell area of lymph nodes. The patient presented with superior vena caval obstruction due to a mediastinal mass, followed by lymph node enlargement and skin lesions leading to death within 4 months. The tumor cells lacked immune markers for lymphocytic cells. They showed Ia-like antigens and high adenosine triphosphatase activity, while acid phosphatase and alpha-naphthyl acetate esterase activity was absent. Their fine morphology was strikingly similar to that of interdigitating cells. A combination of these data led us to the conclusion that this tumor represents a specific subtype of the tumors derived from the mononuclear phagocyte system, namely a sarcoma of interdigitating cells.