Medulloblastoma in an adult suggestive of external granule cells as its origin: a histological and immunohistochemical study.

We describe a rare case of medulloblastoma in an adult woman with histological findings suggesting an origin for this neoplasm in the external granular layer or its remnants. The patient presented with cerebellar dysfunction, and neuroimaging revealed a right cerebellar mass lesion. Pathological examination of the operative specimen revealed a medulloblastoma with occasional areas of neuronal or glial differentiation. Zic protein was also detected immunohistochemically in the tumor cells. The tumor cells were mainly distributed in the subarachnoid space and extended to the cerebellar parenchyma through the perivascular space to form tumor nodules. A suggestive finding, as concerns the origin of this neoplasm, was that the tumor cells were also spread evenly along the subpial zone of the molecular layer, reminiscent of the cellular architecture of the fetal external granular layer.