Case of primary central nervous system histiocytic sarcoma with prominent proliferation of histiocytic cells between the trabeculae of reactive glial cells.

Histiocytic sarcoma (HS) is an extremely rare malignant neoplasm that exhibits morphologic and immune-phenotype evidence of histiocytic differentiation. The disease most commonly involves the lymph nodes, gastrointestinal tract, skin, and soft tissue, as well as in the central nervous system (CNS) being relatively rare. Here we report a case of primary CNS HS with unusual histopathological characteristics. A 65-year-old woman presented with CNS HS in the left frontal lobe region, showing two distinct histological patterns. Approximately half of the lesion displayed histological characteristics typical of HS, including diffuse invasion of large round-to-ovoid pleomorphic cells, with mitotic figures (Ki-67 index: 30%) and coagulative necrotic foci. The other half exhibited prominent proliferation of histiocytic cells between the trabeculae of reactive glial cells, with rare mitotic figures (Ki-67 index: < 1%) and no necrotic foci. There were transitions between two morphologies. The HS tumor cells and the histiocytic cells between the trabeculae of reactive glial cells possessed nearly identical histomorphologic and immunophenotypic features, although the HS tumor cells showed a more pronounced degree of cytologic atypia and mitotic activity. To our knowledge, this is the first reported case of HS with prominent proliferation of the histiocytic cells between the trabeculae of reactive glial cells. Here we present the detailed histological, immunohistochemical, and molecular findings. Investigating cases of HS may provide insight into the pathogenesis of this disease.