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 oligodendrogliomas which essentially appear to be glial neoplasms with overwhelming features of glioblastoma multiforme (GBM) arising from known lower grade oligodendrogliomas 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 oligodendrogliomas. 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 oligodendroglioma 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 oligodendroglioma. Allelic losses on 1p and 19q, either separately or combined, are more common in classic oligodendrogliomas than in either astrocytomas or oligoastrocytomas.
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". Oligodendrogliomas may invade preferentially around vessels or under the pial surface of the brain.Oligodendrogliomas must be differentiated from the more common astrocytoma. Non-classical variants and combined tumors of both oligodendroglioma 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 oligodendrogliomas 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. Oligodendrogliomas are generally dichotomized into grade II (low grade) tumor. Unfortunately, the WHO guidelines include subjective criteria in grade II.