Therefore, it was recommended that the diagnosis of PNET be used for cerebellar medulloblastomas and other indistinguishable neoplasms found in other CNS locations. There may be an embryologic justification for grouping these tumors together with pineoplastoma and ependymoblastoma. The incidence curve reaches a plateau between ages three to eight years, with a peak at age eight, but the tumor has been found in patients of all ages, from newborns to adults in the seventh decade. A second small peak is reported at age 30. There may be a rare familial occurrence.
Exact etiology is unknown.
Several theories exist. Stevenson and Echlin proposed that medulloblastoma arises from remnants of the fetal external granular layer present in the infant cerebellum and then moves through the molecular layer to form the internal granular layer. This theory can be reconciled with that of Raaf and Kernohan who suggested that the tumor arises from cells rests in the inferior medullary velum . There is evidence that the external granular cells originate in the neuroepithelial roof of the fourth ventricle and migrate laterally to form the external granular layer. Recent immunohistochemistry results suggest that medulloblastomas arise from more than one cell of origin.