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A rounded mass is centred on the vermis of the cerebellum, demonstrating only mild enhancement but prominent restricted diffusion, implying high cellularity.
In this location, the diagnosis most likely represents a medulloblastoma.
The patient went on to have surgery.
MICROSCOPIC DESCRIPTION: Paraffin sections show fragments of a densely hypercellular tumor. This is composed of cells with small round hyperchromatic nuclei with delicate processes, arranged in diffuse sheets. A vague nodularity is noted in some areas. Moderate numbers of mitotic and apoptotic figures are noted. There is no microvascular proliferation and no necrosis is seen.
Immunohistochemistry shows strong nuclear staining in tumor cells for INI-1 and strong cytoplasmic staining for beta-catenin and CD56. There is patchy weak staining for synaptophysin and nestin. No staining for CD3, CD20, GFAP, TTF-1, pancytokeratin AE1/AE3, cytokeratin CAM5.2 or epithelial membrane antigen (EMA) is seen in tumor cells.
p53 appears to be wild type.
FINAL DIAGNOSIS: Medulloblastoma (WHO Grade IV).
Unfortunately, immunohistochemistry to determine molecular sub-type is unavailable.
Although we think of medulloblastomas as being pediatric tumours, they are not uncommonly found in young adults. Then, they are typically located in the hemisphere and are of SHH subtype. They can, however, also occur in the vermis.