Previously unrecognized pattern of central nervous system hemangiopericytoma with pseudoglandular spaces.

Central nervous system hemangiopericytoma (HPC) is an uncommon extraaxial tumor with controversial terminology, histogenesis, and grading. Herein, we report a unique example in a 64-year-old woman with a 3.2 cm extra-axial right frontal lobe mass. Histologic sections showed an HPC with increased mitotic activity and elevated proliferation index, interspersed with gland-like spaces lined by epithelioid cells with apical blebs and filled with proteinaceous material. Both spindled and epithelioid cells expressed BCL-2, CD99, and nuclear STAT6, but were negative for CD34, AE1/AE3, CAM5.2, and epithelial membrane antigen. A diagnosis of anaplastic HPC with pseudoglandular spaces was rendered. To our knowledge, this is the first pathologically and molecularly well-documented example in the literature. This pattern has only been reported once in 1983 and is not widely known. Differential diagnosis included synovial sarcoma, anaplastic meningioma with epithelial metaplasia, malignant peripheral nerve sheath tumor, and metastatic carcinosarcoma. Accurate diagnosis depends on recognition of this pattern, along with appropriate immunohistochemical and molecular work-up.