Oligodendrogliomas

ALSO KNOWN AS: Glial brain tumors, OD

RELATED CONDITIONS: Elevated intracranial pressure, personality changes, neurological deficits

DEFINITION: Oligodendrogliomas are a type of glial tumor that occurs in the brain and is predominantly composed of oligodendroglial cells. There are several kinds of glial cells in the central nervous system, but each supports nerve cells (neurons). The oligodendrocytes are glial cells that coat the axons in the central nervous system with an insulating membrane called myelin. Without myelin, the nerve impulses flowing down the axon lose efficiency.

Oligodendrogliomas are among the most common glial neoplasm tumors. They usually originate in the cerebral hemispheres (predominantly the frontal lobe) but can also arise in the cerebellum, brain stem, and spinal cord on rare occasions. Although they are typically considered a rare neoplasm, modern diagnostic techniques, such as genotyping, indicate the condition is more common than once thought.

Oligodendroglioma neoplasms often contain anaplastic cells, primitive, undifferentiated cells that are a distinguishing feature of emerging embryonic cells and are often seen in malignant neoplasms. Tumors with anaplastic cells are called anaplastic oligodendrogliomas.

Risk factors: Evidence suggests that genetic mutations in chromosomes play a role in the development of oligodendrogliomas. 1p/19q co-deletion and IDH1 or IDH2 mutation cause oligodendrogliomas. These genetic changes are not inherited. Primary brain tumors such as oligodendrogliomas affect men about twice as often as women, and White Americans are at higher risk than Black Americans. Although the median age of diagnosis is between forty and fifty, oligodendroglioma can occur at any age.

Etiology and the disease process: Oligodendrogliomas evolve from primitive precursor glioma cells. The tumor begins as a slow-growing, mixed tumor, combining elements of astrocytes (another type of glial cell), neurons, and oligodendroglial cells. The tumor consists of poorly defined anaplastic components, mature oligodendroglial cells, or a mixture. It is impossible to predict how long it will take for patients to experience symptoms because although oligodendrogliomas usually grow quite slowly, they can quickly evolve into an aggressive malignancy and shorten the crisis point from upwards of thirty years to only a few months. When the neoplasm exhibits features of oligodendroglioma cells with astrocytic components, it is called oligoastrocytoma, but this is less common.

Incidence: Between 5 and 15 percent of all gliomas, or tumors from glial cells, are oligodendrogliomas, which comprise 3 to 4 percent of all brain tumors. Less than 25,000 people are diagnosed with oligodendrogliomas annually, most of whom are between forty and fifty.

Symptoms: Symptoms depend on the size, location, invasiveness, and rate of growth of the tumor. Warning signs will vary but may include seizures, vomiting, fatigue, headache, numbness, pain, ataxia, sensations of tingling or burning, and changes in temperature and taste sensitivity. The most common symptom is a seizure, with 80 percent of patients experiencing at least one seizure. Likewise, worsening headaches occur in up to half of patients. Focal symptoms, or symptoms that result from the tumor impacting a localized area of the brain, include muscle weakness, paralysis, aphasia, hearing or vision deficits, memory problems, concentration deficits, and trouble thinking.

Screening and diagnosis: Imaging studies and biopsy examination are central to oligodendroglioma diagnoses. Laboratory tests are helpful only in excluding other causes and determining overall health. Magnetic resonance imaging (MRI) is performed with and without the radiocontrast medium gadolinium. Computed tomography (CT) scans can reveal details missed in MRI studies, and sometimes immunohistochemical markers further help establish a diagnosis. However, only biopsy results are definitive. Following biopsy, pathologists grade oligodendrogliomas according to the presence or absence of anaplastic features according to the World Health Organization's (WHO's) grading scale. Oligodendrogliomas WHO grade 2 are low-grade oligodendrogliomas that respond well to treatments and grow slowly. Oligodendrogliomas WHO grade 3 are malignant, high-grade oligodendrogliomas that are aggressive and difficult to treat. Grade 3 oligodendrogliomas were formerly called anaplastic oligodendrogliomas.

Treatment and therapy: Treatment depends on the patient’s symptoms and the location and nature of the tumor. Like most cancers, the three mainstays of treatment are chemotherapy, radiation, and surgery. Patients may receive radiation therapy, chemotherapy, or both, or be recommended to watchful waiting following surgery. The PCV regimen—a combination of procarbazine, lomustine, and vincristine—is an effective treatment, though some healthcare professionals recommend temozolomide because of its lowered risk of side effects. Targeted radiation is also commonly and effectively used to treat oligodendrogliomas. Surgery may be appropriate based on the location, size, and type of the cancer. Often, the tumor is poorly defined, making complete resection impossible. In the past, when surgery completely removed the cancer, further treatment was believed to be unnecessary. However, several years following total resection, many patients experience a recurrence of the tumor at the surgical site. In the event of relapse, chemotherapy is the treatment of choice. Repeat radiation carries several risks that may compromise brain function. Steroids and anticonvulsants may help manage symptoms.

Prognosis, prevention, and outcomes: The prognosis depends on the growth rate of the tumor, its location, and its pressure effects inside the skull. Some specific prognostic indicators have been noted—older age at the time of diagnosis, a neurological deficit, or a central location in a child’s brain usually results in a shorter survival time. Though oligodendrogliomas are brain tumors, their survival rate and outcomes are better than other similar cancers, with a five-year survival rate of around 75 percent.

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