Microscopic appearance Low magnification micrograph of an<unk>  showing the characteristic, small, branching, chicken wire-like blood vessels. H&E stain.<unk> s cannot currently be differentiated from other brain lesions solely by their clinical or radiographic appearance. As such, a brain biopsy is the only method of definitive diagnosis.<unk> s recapitulate the appearance of the normal resident oligodendroglia of the brain. (Their name derives from the Greek roots 'oligo' meaning " few" and 'dendro' meaning "trees".) They are generally composed of cells with small to slightly enlarged round nuclei with dark, compact nuclei and a small amount of eosinophilic cytoplasm. They are often referred to as "fried egg" cells due to their histologic appearance. They appear as a monotonous population of mildly enlarged round cells infiltrating normal brain parenchyma and producing vague nodules. Although the tumor may appear to be vaguely circumscribed, it is by definition a diffusely infiltrating tumor. Classically they tend to have a vasculature of finely branching capillaries that may take on a "chicken wire" appearance. When invading grey matter structures such as cortex, the neoplastic oligodendrocytes tend to cluster around neurons exhibiting a phenomenon referred to as "perineuronal satellitosis".<unk> s may invade preferentially around vessels or under the pial surface of the brain.<unk> s must be differentiated from the more common astrocytoma. Non-classical variants and combined tumors of both<unk>  and astrocytoma differentiation are seen, making this distinction controversial between different neuropathology groups. Molecular diagnostics may make this differentiation obsolete in the future. Other glial and glioneuronal tumors with which they are often confused due to their monotonous round cell appearance include pilocytic astrocytoma, central neurocytoma, the so-called dysembryoplastic neuroepithelial tumor, or occasionally ependymoma. Histopathological grading The histopathologic grading of<unk> s is controversial. Currently, the most commonly used grading schema is based on the year 2007 World Health Organization (WHO) guidelines. An updated classification is in progress.<unk> s are generally dichotomized into grade II (low grade) tumor. Unfortunately, the WHO guidelines include subjective criteria in grade II. In addition, the presence of low mitotic activity, vascular proliferation and necrosis, including pseudopallisading necrosis, is insufficient by itself to elevate the grade of this tumor. This leads to inevitable interobserver variability in diagnosis by pathologists. The ultimate responsibility for making treatment decisions and interpretation of this diagnosis lies with the oncologist in consultation with the patient and their family. It has been proposed that WHO guidelines should contain a category for grade IV<unk> s which essentially appear to be glial neoplasms with overwhelming features of glioblastoma multiforme (GBM) arising from known lower grade<unk> s or GBM with a significant proportion of oligodendroglial differentiation. The diagnostic utility of this latter category is uncertain as these tumors may behave either like glioblastoma or grade III Anaplastic<unk> s. As such,this is an exceptionally unusual diagnosis. The updated WHO guidelines published in 2007 recommends classifying such tumors for the time being as 'glioblastoma with<unk>  component'. It remains to be established whether or not these tumors carry a better prognosis than standard glioblastomas. Molecular genetics By far, the most common structural deformity found is co-deletion of chromosomal arms 1p and 19q. The high frequency of co-deletion is a striking feature of this glial tumour and is considered as a "genetic signature" of<unk> . Allelic losses on 1p and 19q, either separately or combined, are more common in classic<unk> s than in either astrocytomas or oligoastrocytomas. In one study, classic<unk> s showed 1p loss in 35 of 42 (83%) cases, 19q loss in 28 of 39 (72%), and these were combined in 27 of 39 (69%) cases; there was no significant difference in 1p/19q loss of heterozygosity status between low-grade and anaplastic<unk> . 1p/19q co-deletion has been correlated with both chemosensitivity and improved prognosis in<unk> s. The gene products lost as a consequence of this codeletion may include mediators of resistance to genotoxic therapies. Alternatively, 1p/19q loss might be an early oncogenic lesion promoting the formation of glial neoplasms, which retain high sensitivity to genotoxic stress. Most larger cancer treatment centers routinely check for the deletion of 1p/19q as part of the pathology report for<unk> s. The status of the 1p/19q loci can be detected by FISH, loss of heterozygosity (LOH) analysis or virtual karyotyping. Virtual karyotyping has the advantage of assessing the entire genome in one assay, as well as the 1p/19q loci. This allows assessment of other key loci in glial tumors, such as EGFR and TP53 copy number status. Whereas the prognostic relevance of 1p and 19q deletions is well established for mixed oligoastrocytomas, the prognostic relevance of the deletions for low-grade gliomas is more controversial. In terms of low-grade gliomas, a recent study also suggests that 1p/19q co-deletion may be associated with a (1;19)(q10;p10) translocation which, like the combined 1p/19q deletion, is associated with superior overall survival and progression-free survival in low-grade glioma patients.<unk> s show only rarely mutations in the p53 gene, which is in contrast to other gliomas. Epidermal growth factor receptor amplification and whole 1p/19q codeletion are mutually exclusive and predictive of completely different outcomes, with EGFR amplification predicting poor prognosis. There is a strong correlation between 1p/19q codeletion and the expression of proneural genes, suggesting that gliomas with a 1p19q codeletion represent a subgroup of proneural gliomas.
Oligodendroglioma
Neurothekeoma
Pilocytic astrocytoma
Astroblastoma
Ependymoma
Astrocytoma
Glioblastoma
Giant-cell glioblastoma