Definition: `A tumor consisting of neoplastic cells that resemble the undifferentiated cells of the primitive medullary tube; medulloblastomas are usually located in the vermis of the cerebellum, and may be implanted discretely or coalescently on the surfaces of the cerebellum, brainstem, and spinal cord; they comprise approximately 3% of all intracranial neoplasms, and occur most frequently in children; the neoplastic cells are compactly arranged, rounded or ovoid, with hyperchromatic nuclei and relatively scant cytoplasm, and lie in small and poorly defined groups, or, occasionally, in a pseudorosette pattern (Homer-Wright rosette). A type of primitive neuroectodermal tumor.
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