The term PNET (primitive neuroectodermal tumor) refers to a group of highly malignant tumors arising from neuroepithelial precursors and sharing a undifferen-tiated and mitotically active histology. PNETs found in the posterior fossa have traditionally been called medulloblastomas. Other tumors that fall under the rubric of PNET include pineoblastomas, Ewing's sarcoma and neuroblastoma. Eighty percent of these tumors arise during the first 2 decades of life (median age 5-7 years) and are more common in boys. The overall incidence among children is approximately 5 per million per year. Medulloblastomas occur sporadically and no environmental factors are known to predispose to oncogenesis.

The histological appearance of medulloblastoma is that of a 'small blue cell tumor.' In other words, the tumor is composed of densely packed cells that have scant cytoplasm and hyperchromatic nuclei. Although typically undifferentiated in appearance, astrocytic, neuronal, ependymal and/or mesodermal features can occur.

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