Classification of brain tumors in children

In 1926, Bailey and Cushing developed a classification of brain tumors based on the general concept of oncology. According to this concept, tumors develop from cells at different stages of morphological and functional development. The authors suggested that each stage of glial cell development corresponds to its own tumor. Most modern morphological and histological classifications are based on modifications of the work of Bailey and Cushing.

The modern histological classification of CNS tumors (WHO, 1999) more fully reflects the histogenesis and degree of malignancy of a number of neoplasms due to the use of the latest methods in neuromorphology, including immunohistochemistry and molecular genetic analysis. CNS tumors in children are characterized by heterogeneity of cellular composition. They contain neuroectodermal, epithelial, glial and mesenchymal components. Determination of the histological type of tumor is based on the identification of the predominant cellular component. Below is the WHO classification of 1999 with abbreviations.

Histological variants of CNS tumors

• Neuroepithelial tumors.
  • Astrocytic tumors.
  • Oligodendroglial tumors.
  • Ependymal tumors.
  • Mixed gliomas.
  • Tumors of the choroid plexus.
  • Glial tumors of unknown origin.
  • Neuronal and mixed neuronal-glial tumors.
  • Parenchymatous tumors of the pineal gland.
  • Embryonic tumors.
• Tumors of the cranial and spinal nerves.
  • Schwannoma.
  • Neurofibroma.
  • Malignant tumor of the peripheral nerve trunk.
• Tumors of the meninges.
  • Meningoepithelial cell tumors.
  • Mesenchymal non-meningoepithelial tumors.
  • Primary melanocytic lesions.
  • Tumors of unknown histogenesis.
• Lymphomas and tumors of hematopoietic tissue.
  • Malignant lymphomas.
  • Plasmacytoma.
  • Granulocytic sarcoma.
• Germ cell tumors.
  • Germinoma.
  • Embryonic cancer.
  • Yolk sac tumor.
  • Choriocarcinoma.
  • Teratoma.
  • Mixed germ cell tumors.
• Tumors of the sella turcica region.
  • Craniopharyngioma.
  • Granular cell tumor.
• Metastatic tumors.

This classification provides for the definition of several degrees of malignancy of astrocytic and ependymal tumors. The following criteria are used:

• cellular pleomorphism;
• mitotic index;
• nuclear atypia;
• necrosis.

The degree of malignancy is determined as the sum of the four listed histological features.

Phenotypic classification

In addition to purely morphological and histogenetic concepts, there is a phenotypic approach to classifying CNS tumors. Immunohistochemical and molecular methods are used as an addition to standard light and electron microscopy, which allows for a more accurate and objective determination of the cell type of a brain tumor. A number of tumors are phenotypically polymorphic, since they consist of tissues of different genesis. Immunohistochemical examination of an atypical teratoid-rhabdoid tumor revealed that rhabdoid cells often express epithelial membrane antigen and vimentin, and less often, smooth muscle cell actin. These cells can also express glial fibrillary acidic protein, neurofilaments, and cytokeratins, but never express desmin and markers of germ cell tumors. Small embryonic cells express markers of neuroectodermal differentiation and desmin inconsistently. Mesenchymal tissue expresses vimentin, and epithelium expresses cytokeratins of various molecular weights. Teratoid-rhabdoid tumors have significant proliferative activity, the labeling index of the proliferative marker Ki-67 in the vast majority of cases exceeds 20%.

Classification of brain tumors in children

Brain tumors in children differ from those in adults. Among adults, supratentorial tumors, mainly gliomas, predominate significantly. Most neoplasms in children are located infratentorially, about 20% are undifferentiated embryonic tumors. The prognosis is determined by the biological nature of the tumor and surgical accessibility, so with different locations of similar histological tumors, the prognosis may be different.

Of the many histological types of brain tumors among children, the most common group is embryonic tumors, consisting of poorly differentiated neuroepithelial cells. According to the 1999 WHO classification, this group includes medulloblastoma, supratentorial primitive neuroectodermal tumor, atypical teratoid-rhabdoid tumor, medulloepithelioma, and ependymoblastoma. The vast majority of tumors are represented by the first three histological types.

The identification of embryonic tumors is based on the following:

• they occur exclusively in childhood;
• have a uniform clinical course, characterized by a pronounced tendency to leptomeningeal spread, which necessitates prophylactic craniospinal irradiation;
• Most tumors of this group (medulloblastoma, supratentorial primitive neuroectodermal tumor, and ependymoblastoma) consist predominantly of primitive or undifferentiated neuroepithelial cells, although they also contain cells morphologically resembling neoplastic astrocytes, oligodendrocytes, ependymal cells, neurons, or melanocytes (some tumors may contain smooth or striated myofibrils, fibrocollagenous tissue).

Tumors with the above features are typical for the cerebellum (medulloblastoma). However, histologically identical tumors can also arise in the cerebral hemispheres, pituitary gland, brainstem, and spinal cord. In this case, they are designated by the term "supratentorial primitive neuroectodermal tumor". The division of medulloblastoma and primitive neuroectodermal tumors is based on their molecular and biological properties. The group of embryonic tumors includes, due to the high risk of their leptomeningeal spread, atypical teratoid-rhabdoid tumors, recently isolated as a separate histological variant. Histologically, these neoplasms differ from embryonic tumors of the central nervous system. They consist of tissues of various genesis - large rhabdoid cells in combination with areas of neuroectodermal, mesenchymal and epithelial origin. In some cases, the tumor may consist only of rhabdoid cells; two-thirds of tumors have a pronounced small-cell embryonic component.

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