Walsh & Hoyt: Primitive Neuroectodermal Tumors

Despite attempts to classify all primitive neuroectodermal tumors (PNETs) according to their presumed cell of origin, there remain tumors that, although neuroectodermal in character, are so poorly differentiated that their cell of origin is impossible to identify. They are highly cellular and occur most frequently in children. When they occur in the cerebellum, they are called medulloblastomas, but histologically similar neoplasms also occur in the cerebral hemispheres, spinal cord, and even in the orbit. The cerebral examples of this type of neoplasm are often designated as primary cerebral neuroblastomas, particularly when they occur in adults. Many authors, however, prefer to call all of these lesions PNETs. This term raises several issues, primarily whether primitive tumors that occur in the pineal region and in the cerebellum should be included. It has been suggested that all undifferentiated neuroepithelial tumors of the CNS should be included in this classification. The following nomenclature is proposed for undifferentiated neuroepithelial round cell tumors based on light microscopic, ultrastructural, and immunocytochemical evidence: (a) PNET, (b) PNET with glial differentiation, (c) PNET with ependymal differentiation, (d) PNET with neuronal differentiation, and (e) PNET with bi- or multipotential differentiation. Other investigators prefer to retain the terms medulloblastoma, pineoblastoma, and neuroblastoma when appropriate by location. The 1994 WHO classification used the term PNET in a limited fashion to describe small-cell tumors showing divergent differentiation, including medulloblastomas and morphologically similar tumors in the supratentorial compartment. The 2002 WHO classification also differentiates medulloblastoma from supratentorial primitive neuroectodermal tumors (SPNETs), a category which includes neuroblastoma and ganglioneuroblastoma.