Intracranial tumors comprise approximately 2% of all adult cancers, but form a larger fraction within the group of childhood tumors. Gliomas account for approximately 60% of all intracranial tumors and are classified according to the suggested cell of origin, differentiation and malignancy grade. The prognosis for high-grade gliomas is poor due to limited possibilities of curative treatment.
Gliomas are tumors of neuroepithelial tissue and comprise a complex and heterogeneous group of tumors representing counterparts to various normal inhabitant cells of the central nervous system (CNS). The most common form of glioma is astrocytoma, representing approximately one third of all gliomas.
Astrocytomas are defined based on morphological features such as cellularity, nuclear atypia, mitotic rate, endothelial proliferation and necrosis, and assigned to grades I-IV according to the current WHO classification system. These include pilocytic astrocytoma (Grade I), astrocytoma (Grade II), anaplastic astrocytoma (Grade III), and glioblastoma (Grade IV). The various forms of glioma are highly variable and several phenotypically different cell types exist, including gemistocytic glioma cells. Gemistocytic cells resemble a morphological alteration that can also be found in reactive astrocytes and is characterized by eosinophilic staining of a large, swollen cytoplasm.
Approximately 15% of gliomas are oligodendrogliomas. Histologically, oligodendrogliomas commonly show uniform cell architecture with increased numbers of delicate blood vessels. The tumor cell nuclei are mainly round and regular, and often surrounded by an artifactual perinuclear clearing that results in the so called "fried egg" appearance. High-grade oligodendrogliomas (anaplastic oligodendrogliomas) are recognized by features such as increased cellularity, mitotic activity and nuclear pleomorphism, as well as necrosis and endothelial proliferation.
Additional forms of glioma include mixed gliomas such as oligoastrocytoma and ependymal tumors. Examples of other neuroepithelial tumors that grow within the CNS are neuronal and mixed neuronal-glial tumors, as well as embryonal tumors including neuroblastoma and medulloblastoma.
The distinction between different forms of brain tumors is mainly based on morphological features, but immunohistochemistry plays an important role to distinguish between different tumor types, in particular when the tumor is poorly differentiated. In neuropathological diagnostics, antibodies directed towards proteins such as Glial Fibrillary Acidic Protein, Synaptophysin, EGFR, p53 and the proliferation marker Ki-67 are commonly used.