Solitary fibrous tumor (hemangiopericytoma)

Discussion:

Features are consistent with a solitary fibrous tumor of the dura (then known as a hemangiopericytoma). The diagnosis was confirmed after resection. 

Histology

Microscopic Description:

Sections show a densely cellular neoplasm composed of closely packed, randomly oriented cells interspersed by numerous slit-like vascular channels. Scattered through are several wide and branching thin-walled channels with a "staghorn" appearance. The tumor cells have scant cytoplasm with ill-defined borders and display oval, plump, or elongated nuclei with evenly dispersed chromatin. Only a few mitotic figures are identified. MIB-1 immunostaining shows only a few scattered cells with positive nuclei. Throughout the tumor are patches of hypocellularity. The reticulin stain shows a rich reticulin network that often typically invests individual cells. There are no anaplastic features such as necrosis or high mitotic activity. The tumor cells are strongly positive for vimentin and negative for EMA and S-100. There is patchy strong immunoreactivity of the tumor cells for CD34. Endothelial cells lining the sinusoidal vessels are strongly positive for both Factor-VIII-R:Ag and CD34. The tumor is firmly attached to the dura. In sections B9 and B14, small fragments of bone are incorporated within the tumor, indicating bone invasion. Frequently encountered throughout the tumor are mast cells.

Final Diagnosis: hemangiopericytoma.

Note: Due to molecular/genetic similarities hemangiopericytomas are no longer recognized as distinct entities. Instead, they represent cellular higher-grade solitary fibrous tumors. In the 2016 revised 4th Edition of the WHO classification of CNS tumors, they were grouped together under one diagnosis (solitary fibrous tumor/hemangiopericytoma), and in the 2021 5th edition the term hemangiopericytoma was dropped entirely. 

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