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A large vividly enhancing but heterogeneous mass is seen on the left, abutting / arising form the tentorium and invaginating / extending into the undersurface of the occipital lobe. It is surrounded by cysts. ADC demonstrates facilitated diffusion compared to brain. Prominent flow voids are present.
The patient went on to have a resection.
Paraffin sections show a densely hypercellular tumor. Tumor cells have moderately pleomorphic round, oval and angulated vesicular nuclei. Cytoplasmic boundaries are poorly defined. Tumor cells are aggregated in solid sheets with no specific architectural pattern discernible. However, in several areas, small caliber open vascular spaces are noted. These are lined by unremarkable endothelium. A moderately fibrous stroma is noted in several areas of the tumor. A very occasional mitotic figure is identified. There are no areas of necrosis. No evidence of invasion into brain parenchyma is seen.
Immunohistochemistry shows strong staining of tumor cells for CD34 and bcl-2. There is also moderate nuclear staining for progesterone receptor (PgR). No staining for epithelial membrane antigen (EMA), cytokeratin AE1/AE3, GFAP, ALK-1 or tyrosinase is seen in tumor cells. The features are of hemangiopericytoma (WHO Grade II). The topoisomerase labeling index is <1%.
FINAL DIAGNOSIS: Hemangiopericytoma (WHO Grade II)