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Cellular Classification of Central Nervous System (CNS) Embryonal Tumors

The classification of a childhood brain tumor is based on both the histopathologic characteristics of the tumor and its location in the brain.[1,2,3] The histopathological classification of childhood central nervous system (CNS) embryonal tumors remains somewhat controversial. These tumors all develop on the background of an undifferentiated round cell tumor but show a variety of divergent patterns of differentiation. Although it has been proposed that these tumors be merged under the term primitive neuroectodermal tumor, histologically similar tumors in different locations in the CNS demonstrate different genetic alterations.[4,5,6,7,8] In the 2007 World Health Organization (WHO) classification, embryonal tumors include the following:[2,9]

  • Medulloblastoma.
  • CNS primitive neuroectodermal tumors (PNETs).
  • Medulloepithelioma.
  • Ependymoblastoma.
  • Atypical teratoid/rhabdoid tumor. Refer to the PDQ summary on Childhood Central Nervous System Atypical Teratoid/Rhabdoid Tumors for more information.

Pineoblastoma, a tumor histologically similar to CNS embryonal tumors, is reviewed in this summary, although pineoblastoma is grouped with tumors of the pineal region in the WHO classification.

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By definition, medulloblastomas must arise in the posterior fossa.[1,2,3] Five different subtypes of medulloblastoma are recognized by the WHO classification and includes the following:

  • Classic.
  • Desmoplastic/nodular.
  • Medulloblastoma with extensive nodularity.
  • Large cell.
  • Anaplastic.[1]

Significant attention has been focused on medulloblastomas that display anaplastic features, including increased nuclear size, marked cytological pleomorphism, numerous mitoses, and apoptotic bodies.[10,11] Classification is a complicated matter because most medulloblastomas have some degree of anaplasia, foci of anaplasia may appear in tumors with histologic features of both classic and large cell medulloblastomas, and there is significant overlap between the anaplastic and large cell variant.[10,11,12,13] The incidence of medulloblastoma with desmoplastic variant is higher in infants, is less common in children, and increases again in adolescents and adults. The desmoplastic variant subtype is different from medulloblastoma with extensive nodularity; the nodular variant having an expanded lobular architecture. The nodular subtype occurs almost exclusively in infants and carries an excellent prognosis.[13]

CNS PNETs generally arise in the cerebrum or suprasellar region, but may arise in the brain stem and spinal cord.[14] According to the 2007 WHO classification, tumors demonstrating areas of distinct neuronal differentiation are termed cerebral neuroblastomas and, if ganglion cells are also present, ganglioneuroblastomas. The pineoblastoma is histologically similar to the medulloblastoma; however, according to the WHO, its histogenesis is linked to a pineal cell, the pineocyte. Histologically different from the pineocyte, a pineal parenchymal tumor of intermediate differentiation showing elements of pineoblastoma and pineocytoma is recognized, although its natural history is variable and poorly characterized.[1,2] Genome-wide characterization of PNETs and pineoblastomas has demonstrated substantial molecular heterogeneity among these tumors.[8]

Both medulloepithelioma and ependymoblastoma are identified as histologically discrete tumors within the WHO classification system.[9,15] Medulloepithelioma and ependymoblastoma tumors are rare and tend to arise most commonly in infants and young children. Medulloepitheliomas, which histologically recapitulate the embryonal neural tube, tend to arise supratentorially, primarily intraventricularly, but may arise infratentorially, in the cauda, and even extraneural, along nerve roots.[9,15]

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