Oligodendroglial cell tumor is a relatively rare neuroepithelial tumor. In the past, it was thought to account for 2%-12% of gliomas and 1.3%-3.8% of intracranial tumors. However, with the improvement of pathological diagnosis in recent years, oligodendroglial tumors have been found to be a relatively rare neuroepithelial tumor. It has been found that oligodendroglial cell tumors are often mistaken for astrocytomas or glioblastomas. The actual proportion of gliomas is about 25%. The tumor tissue grows infiltratively in all directions in the white matter of the brain. The common sites of growth are the frontal lobe, temporal lobe and corpus callosum. When frontal lobe tumors invade the corpus callosum, they may extend to the contralateral cerebral hemisphere, forming bilateral hemispheric tumors. Some tumors may extend to the temporal lobe or occipital lobe, and may also invade the cerebral cortex. If the tumor occurs in the posterior wall of the third ventricle, it may extend to the tegmentum and pineal area; if it occurs in the thalamus, it may extend to the midbrain and other brainstem structures. The disease mainly occurs in adults, and the peak incidence is between 3O and 40 years old. There is a small peak between the ages of 6 and 12 years, with more males than females. The male to female ratio is about 2:1. Oligodendroglioma can be classified according to the WHO 2007 classification criteria: oligodendroglioma grade II, anaplastic oligodendroglioma grade III, oligo- astrocytoma grade II. astrocytoma grade II) and anaplastic oligo-astrocytoma grade III. The tumor grows slowly and can last for more than 10 years, often manifesting clinically as epilepsy or localized paralysis. If the tumor cells are markedly heterogeneous, the growth is rapid and the prognosis is poor. Pathology (a) Oligodendroglioma Oligodendroglioma is a tumor arising from oligodendrocytes. It is a neuroglioma composed of cells rich in round and honeycomb architecture. The tumor is mainly located in the white matter under the cerebral cortex and can be as big as a duck egg or a male fist. It is gray or gray-red in color, solid in texture, and can be seen as gritty. It is a calcium salt deposited (neuro)glioma. Calcium salt deposition can be seen in the peripheral part of the tumor, and the large size can affect the gray matter, and there can be hemorrhage and cystic changes, but necrosis is not common. 2.Microscopic view The most prominent feature of tumor cells is cell swelling. The cytoplasm is swollen and degenerated to be hollow, the cytoplasmic protrusion is retracted, the cell boundary is clear, and the nucleus is located in the hollow cytoplasm. The nucleus is located within the hollow cytoplasm. The tumor cells are round in shape with a thin film around the cytoplasmic edge, sometimes connected to neighboring cells in a latticework pattern. On metal-impregnated sections, the cell protrusions are sparse and the nuclei are not colored but appear as translucent dots. The tumor cells are densely arranged and uniform, with roughly equal intercellular distances and sparse interstitium; there are only nearly normal or slightly dilated capillaries with thin, nonproliferative walls and few glial fibers. Calcification is more common than in other gliomas and is one of the diagnostic features of this tumor. Calcification often occurs within the blood vessels, but can also occur in any area of the tumor or even in brain tissue outside the tumor. The size of calcifications varies, from small ones visible only under the microscope to large ones that can occupy most of the tumor. They take various forms, either in irregular plaques or concentric rings, and cystic changes are more common, while necrosis is rare. Immunohistochemical staining is positive for galactosidase, carbonic acid glycosidase isoenzyme CD57 and MBP (basic myelin protein). (B) Mesenchymal oligodendroglioma Mesenchymal oligodendroglioma is also known as malignant oligodendroglioma or adult oligodendroglioma. 1.Sarcoma is similar to oligodendroglioma, but it is larger in size and can be seen to have hemorrhage and necrosis. 2.Microscopic view The basic morphology of interstitial oligodendroglioma is the same as that of oligodendroglioma, except that the tumor grows more actively, with interstitial changes and mild interstitial reaction. The number of tumor cells is increased and dense; the nucleus is enlarged, the chromatin is loose and lightly stained, and the perinuclear halo, although mostly present, is narrow and atypical. Large cell oligodendroglial cells are also classified in this category and are characterized by hypertrophic cytoplasm, eosinophilia, and a lateralized nucleus, maintaining the characteristics of a typical oligodendroglioma. In more malignant oligodendrogliomas, the proliferation is more active, and interstitial changes and interstitial reactions are apparent, with the appearance of pleomorphic cells and tumor giant cells. Molecular biology features Molecular biology is a hot topic in the study of oligodendroglial tumors and has made great progress, as oligodendroglial tumors usually show specific genetic alterations that distinguish them from other types of gliomas. It is also important for the diagnosis and treatment of oligodendroglioma, as well as for the prognosis. The most common genetic alteration is LOH on the long arm of chromosome 19 (19q), with a common deletion region at 19q13.3 and an incidence of 50% to 80%. This is followed by LOH on the short arm of chromosome 1 (1p), with an incidence of 40% to 92%. These two genes may play a role in the early stage of neoplastic transformation in oligodendrogliomas. Grade III oligodendrogliomas have a much higher incidence of LOH on chromosome 9p and/or 1O (approximately 25%), although the odds of LOH on 1p and 19q are almost identical to those of grade II. In addition, some mesenchymal oligodendrogliomas can have concurrent LOH on chromosome 7 in addition to LOH on chromosome 1O with 1p and 19q intact, and these tumors are considered another subgroup of mesenchymal oligodendrogliomas with a worse prognosis than the 1p and 19q LOH subgroups.Gresner et al. studied 22 low-grade oligodendrogliomas (WHO classification grade II) found that 86% of patients had lp/19q LOH, and the same high rate of 1p/1 9q deletion was seen in mesenchymal oligodendrogliomas (WHO classification grade III), which is rare in other types of gliomas, such as astrocytomas. OLIG1/2 is a family gene of oligodendroglial cells. It is expressed not only in mature cells but also in cellular precursors of embryonic neural tube. The expression is independent of the degree of cell differentiation. The positive rate of this gene in oligodendroglioma cells was 76.9%. In contrast, the positive rate was 16.7% in astrocytomas and negative in glioblastomas and normal brain tissue. Therefore, it is important to differentiate oligodendroglioma from other types of glioma. LOH10q/EGFR amplification can be used as a marker of poor prognosis in oligodendroglioma. Imaging There are five characteristic CT features of oligodendroglial tumors: (1) striated or mass-like calcifications, which can be seen in more than 90% of cases; (2) equal or slightly dense lesions; (3) no or mild enhancement; (4) no or mild edema around the tumor; and (5) a tendency to develop in the peripheral parts of the brain and to infiltrate the skull. The cause of calcification is related to calcium deposition in the vessel wall. Most of the tumors show mixed long T1 and long T2 signals on MR examination, especially T2-Wl, and more cords and clusters of low signal areas can be seen in high signal lesions, which are mostly due to calcification and help to diagnose this disease. In the past, glioblastoma with significant enhancement was considered to be interstitial or malignantly progressive. However, oligodendroglioma (grade II) lesions can also show dot or stripe enhancement, which should be differentiated. PET can be used to differentiate low-grade from high-grade gliomas by measuring the tumor’s glucose and amino acid metabolic rate. Low-grade oligodendroglial tumors often show areas of decreased metabolic rate, while high-grade gliomas have increased metabolic rate. Clinical manifestations The clinical symptoms of oligodendroglial tumors are atypical. They vary depending on the site of tumor growth. Among them, epilepsy is the most common symptom of this disease, and the incidence can be as high as 80%. It is the most common symptom among neuroepithelial tumors and is the first symptom in most patients. In addition, hemiplegia, aphasia, visual and field impairment, cognitive and memory impairment are also seen. The symptoms of hypercranial pressure usually appear later. Treatment Oligodendroglial tumors are mainly treated by surgery, and those who cannot be completely excised can be treated with radiation therapy and chemotherapy. For surgical operation, tumors with clear borders and located in non-functional areas should be excised completely or enlarged. For those tumors with extensive infiltration and involving important structures, as many tumors as possible should be removed and fully decompressed while preserving the function. Tumors located in functional areas such as motor and language should be combined with functional brain imaging and electrophysiological monitoring such as intraoperative electrical stimulation to maximize resection of tumors while maximizing protection of related important functions. For recurrent tumors, reoperation should be pursued when the patient’s general condition allows, and postoperative radiation therapy and chemotherapy should be supplemented. Oligodendroglial tumors are relatively sensitive to radiotherapy and chemotherapy, so radiotherapy and chemotherapy can be effective, especially for tumors with LOH of lp/19q. The overall prognosis of oligodendroglial tumors is better than that of astrocytic tumors, with an average survival period ranging from 5.1 to 7.5 years, and it is common for patients with oligodendroglial tumors (grade II) to survive for more than 10 years. With the rapid advances in molecular biology and physical imaging, and the introduction of targeted therapeutic agents, the survival and quality of life of tumor patients will certainly improve to a large extent.