The classification of a childhood brain tumor is based on both the histopathologic characteristics of the tumor and its location in the brain.[1] 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.[2-5] In the 2000 World Health Organization (WHO) classification, embryonal tumors include medulloblastoma, ependymoblastoma, supratentorial primitive neuroectodermal tumor (SPNET), medulloepithelioma, and atypical teratoid/rhabdoid tumor.[1]  [Note: Refer to the PDQ Childhood Central Nervous System Atypical Teratoid/Rhabdoid Tumors summary for more information. Pineoblastoma, a histologically similar tumor, is reviewed in this summary, although pineoblastoma is grouped with the pineal parenchymal tumors in the WHO classification.]

By definition, medulloblastomas must arise in the posterior fossa.[1] Four different subtypes of medulloblastoma are recognized by the WHO: desmoplastic medulloblastoma, large cell medulloblastoma, medullomyoblastoma, and melanocytic medulloblastoma.[1] Recently, significant attention has been focused on medulloblastomas that display anaplastic features, including increased nuclear size, marked cytological pleomorphism, numerous mitoses, and apoptotic bodies.[6,7] 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.[6-8]

SPNETs arise in the cerebrum or suprasellar region. According to the 2000 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]

The pathologic classification of pediatric brain tumors is a specialized area that is undergoing evolution. Immunohistochemical staining is now a routine component of evaluation.[6-8] Molecular genetic profiles are also being incorporated into evaluation and may radically alter classification in the future.[9]

References

1. Kleihues P, Cavenee WK, eds.: Pathology and Genetics of Tumours of the Nervous System. Lyon, France: International Agency for Research on Cancer, 2000. 
   
   
2. Rorke LB: The cerebellar medulloblastoma and its relationship to primitive neuroectodermal tumors. J Neuropathol Exp Neurol 42 (1): 1-15, 1983.  [PUBMED Abstract]
   
   
3. Dehner LP: Peripheral and central primitive neuroectodermal tumors. A nosologic concept seeking a consensus. Arch Pathol Lab Med 110 (11): 997-1005, 1986.  [PUBMED Abstract]
   
   
4. Russo C, Pellarin M, Tingby O, et al.: Comparative genomic hybridization in patients with supratentorial and infratentorial primitive neuroectodermal tumors. Cancer 86 (2): 331-9, 1999.  [PUBMED Abstract]
   
   
5. Nicholson JC, Ross FM, Kohler JA, et al.: Comparative genomic hybridization and histological variation in primitive neuroectodermal tumours. Br J Cancer 80 (9): 1322-31, 1999.  [PUBMED Abstract]
   
   
6. Giangaspero F, Perilongo G, Fondelli MP, et al.: Medulloblastoma with extensive nodularity: a variant with favorable prognosis. J Neurosurg 91 (6): 971-7, 1999.  [PUBMED Abstract]
   
   
7. McManamy CS, Lamont JM, Taylor RE, et al.: Morphophenotypic variation predicts clinical behavior in childhood non-desmoplastic medulloblastomas. J Neuropathol Exp Neurol 62 (6): 627-32, 2003.  [PUBMED Abstract]
   
   
8. Eberhart CG, Kratz J, Wang Y, et al.: Histopathological and molecular prognostic markers in medulloblastoma: c-myc, N-myc, TrkC, and anaplasia. J Neuropathol Exp Neurol 63 (5): 441-9, 2004.  [PUBMED Abstract]
   
   
9. Pomeroy SL, Tamayo P, Gaasenbeek M, et al.: Prediction of central nervous system embryonal tumour outcome based on gene expression. Nature 415 (6870): 436-42, 2002.  [PUBMED Abstract]
   
   

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