Histiocytic Sarcoma : Microscopic
Microscopic Features of Histiocytic Sarcoma: The growth pattern is diffuse with partial or total effacement of the native architecture by a proliferation of high-grade malignant cells resembling histiocytes. In partially involved lymph nodes, the tumor distribution is paracortical. Sinuses may be involved, but there is no preferential involvement of sinuses. The degree of pleomorphism is variable and mitotic activity is increased. The tumor cells have large, eccentrically placed, round to oval nuclei with vesicular chromatin and a single prominent nucleolus. Nuclear outlines may be irregular or multilobated, and multinucleated cells may be present. Nuclear grooves may be seen. There is abundant eosinophilic cytoplasm which is sometimes foamy or vacuolated. Hemophagocytosis and emperipolesis are present in a small number of cases, but are not a requirement for diagnosis. Some areas may appear sarcomatoid and resemble spindle cell sarcomas. The background stroma contains small lymphocytes, plasma cells, eosinophils, and benign histiocytes.