Oligodendroglioma: Understanding Your Pathology Report

By Jason Wasserman MD PhD FRCPC
April 27, 2026

Oligodendroglioma is a type of brain tumor that develops from glial cells, the supporting cells of the central nervous system. It belongs to a larger group of tumors called diffuse gliomas. Diffuse gliomas are infiltrative, which means the tumor cells spread into the normal brain tissue around them and cannot be fully separated from it — unlike another group of gliomas called circumscribed gliomas (such as pilocytic astrocytoma), which have a clear border and can often be completely removed by surgery. In oligodendroglioma, the infiltrative growth pattern has major consequences: even when all visible tumor is removed during surgery, microscopic tumor cells remain behind in the normal-appearing brain. Surgery is often the first step in treatment. Still, additional therapies — which may include a new targeted drug, radiation, or chemotherapy — are used to control the microscopic tumor that surgery cannot reach.

Oligodendroglioma is defined by two specific genetic features that must both be present for the diagnosis: a mutation in one of the IDH genes (IDH1 or IDH2), and the loss of two specific pieces of DNA called 1p/19q codeletion. Without both of these changes, the tumor cannot be classified as an oligodendroglioma. The tumor most often confused with oligodendroglioma is IDH-mutant astrocytoma, which has the IDH mutation but lacks the 1p/19q codeletion. Distinguishing between these two tumors is important because they are treated differently and have different outlooks, even though both are IDH-mutant diffuse gliomas.

Oligodendroglioma typically arises in the cerebral hemispheres, most commonly in the frontal lobes. Less commonly, it can develop in the temporal, parietal, or occipital lobes. Oligodendrogliomas are classified as either World Health Organization (WHO) grade 2 (slower-growing) or grade 3 (faster-growing). Compared with most other types of diffuse glioma, oligodendrogliomas tend to grow more slowly, respond well to treatment, and are associated with substantially longer survival.

This article will help you understand the findings in your pathology report — what each term means and why it matters for your care.

What are the symptoms of oligodendroglioma?

The symptoms of oligodendroglioma depend on the tumor’s size, location, and grade. Because many oligodendrogliomas grow slowly, symptoms often develop gradually over months or years. Some tumors are discovered incidentally when imaging is performed for another reason, such as after a head injury or for unrelated headaches.

Common symptoms include:

• Seizures — the most common first symptom of oligodendroglioma, particularly for tumors in the cortex. Seizures occur in most patients at some point and are usually well-controlled with antiseizure medication.
• Headaches — sometimes worse in the morning or with activities that increase pressure inside the head.
• Weakness or numbness on one side of the body — usually on the opposite side to the tumor.
• Changes in speech, language, or word-finding are common when the tumor is in the left frontal or temporal lobe.
• Vision changes — partial vision loss, double vision, or blurring.
• Changes in thinking, memory, personality, or behavior — especially with frontal lobe tumors. Family members sometimes notice these changes before the patient does.
• Fatigue — can be caused by the tumor itself, by seizures, or by treatment.

What causes oligodendroglioma?

For most people diagnosed with oligodendroglioma, the exact cause is not known. The tumor develops through a series of genetic changes that accumulate in glial cells over time. The two most important changes — the IDH mutation and the 1p/19q codeletion — are required for the diagnosis and together drive the tumor’s growth.

The only well-established environmental risk factor for oligodendroglioma is high-dose ionizing radiation to the head, usually from previous cancer treatment. Other factors commonly discussed in the media — including cell phone use, head injuries, and electromagnetic fields — have not been consistently shown to cause brain tumors. Oligodendroglioma is not contagious and not caused by anything the patient did or did not do.

A small number of oligodendrogliomas develop in the setting of an inherited condition. Inherited conditions are caused by a genetic change that is present in every cell of the body from birth and can be passed from parent to child. Inherited conditions linked to oligodendroglioma include:

• Li-Fraumeni syndrome — caused by a change in the TP53 gene. This syndrome increases the risk of many cancers, including brain tumors, sarcomas, breast cancer, and leukemia.
• Lynch syndrome and constitutional mismatch repair deficiency (CMMRD) — caused by changes in DNA repair genes such as MLH1, MSH2, MSH6, and PMS2. These conditions also increase the risk of colon cancer, endometrial cancer, and several other cancers.
• Turcot syndrome — an older term describing the combination of colon polyps and brain tumors, now recognized as part of familial adenomatous polyposis (APC gene) or Lynch syndrome.

Because most oligodendrogliomas are not associated with an inherited condition, germline (inherited) genetic testing is not routinely recommended. It may be offered when there is a strong family history of related cancers, when a patient is diagnosed at an unusually young age, or when tumor testing suggests a possible inherited change.

How common is oligodendroglioma?

Oligodendroglioma is uncommon. It accounts for about 5% of all primary brain tumors and about 10–15% of all diffuse gliomas. The yearly incidence is approximately 0.4 cases per 100,000 people in North America. Oligodendroglioma is most often diagnosed in adults between 35 and 50 years of age, with a median age at diagnosis of around 40. It is rare in children and uncommon in older adults. Men are slightly more likely than women to develop oligodendroglioma.

How is the diagnosis made?

The diagnosis of oligodendroglioma usually begins when brain imaging — most often magnetic resonance imaging (MRI) — reveals a mass. Oligodendrogliomas typically appear as poorly defined masses in the cortex (the outer layer of the brain) or just below it, most often in the frontal lobes. They often contain calcium specks that appear clearly on CT scans, a feature that can help distinguish oligodendroglioma from other diffuse gliomas. Lower-grade tumors usually show little or no enhancement after intravenous contrast is given. In contrast, higher-grade tumors are more likely to enhance and to show swelling or, occasionally, areas of dead tumor tissue.

The diagnosis is confirmed after a tissue sample is examined under the microscope by a pathologist. In most cases, the tissue is obtained during surgery to remove as much of the tumor as can be safely taken out. Surgery serves two purposes: it reduces the amount of tumor in the brain, which improves outcomes and can relieve pressure, and it provides the tissue needed for diagnosis and molecular testing. When the tumor is in a location where surgery would be too risky, a smaller biopsy is performed instead — typically a stereotactic biopsy, in which a thin needle is guided into the tumor using imaging.

Under the microscope, an oligodendroglioma has a recognizable appearance. The tumor cells have round, uniform nuclei and clear cytoplasm — a “fried egg” pattern that is one of the classic descriptions in pathology. (This pattern is partly an artifact of how the tissue is processed, but it is so characteristic that pathologists rely on it.) The tumor is crisscrossed by a delicate network of small,, branching blood vessels that resemble chicken wire. Specks of calcium (calcifications) are often present, particularly at the edges of the tumor, and are more common in oligodendroglioma than in other diffuse gliomas. The grade of the tumor is determined by how the cells look: grade 2 tumors have few mitotic figures (cells caught in the act of dividing), while grade 3 tumors are more cellular, show more mitotic figures, and may have abnormal new blood vessel growth (microvascular proliferation) or areas of dead tumor tissue (necrosis).

To confirm that the tumor is glial in origin, the pathologist uses immunohistochemistry, a laboratory test that uses antibodies to detect specific proteins in the tumor cells. Oligodendrogliomas typically express GFAP (glial fibrillary acidic protein) and OLIG2, two proteins that confirm the tumor has arisen from glial cells.

The final diagnosis of oligodendroglioma requires both an IDH mutation and 1p/19q codeletion. Without both of these molecular findings, even a tumor that appears to be a classic oligodendroglioma under the microscope cannot be classified as an oligodendroglioma. Molecular testing is therefore an essential part of the diagnosis and is described in detail in the biomarker section below.

WHO grade

The World Health Organization (WHO) assigns tumors of the central nervous system a grade from 1 to 4 that reflects how the tumor is expected to behave. Oligodendroglioma is unusual among brain tumors in that it is only assigned grade 2 or grade 3 — there is no grade 1 oligodendroglioma, and the diagnosis of grade 4 oligodendroglioma is no longer used. The grade is one of the most important pieces of information in the pathology report because it influences treatment decisions and prognosis.

WHO grade 2

Grade 2 oligodendrogliomas grow slowly. Under the microscope, they show a moderate increase in cell number, mild nuclear atypia, and few or no mitotic figures. There is no necrosis and no microvascular proliferation. Although grade 2 is sometimes called “low grade,” these tumors are not harmless — nearly all will eventually progress over time, and most patients will need treatment beyond surgery alone. The good news is that patients with grade 2 oligodendrogliomas often live for many years (often more than 15 years) after diagnosis, especially with modern treatment approaches.

WHO grade 3

Grade 3 oligodendrogliomas (sometimes called anaplastic oligodendrogliomas in older reports) show more aggressive microscopic features: higher cellularity, more nuclear atypia, frequent mitotic figures, and sometimes microvascular proliferation or necrosis. Although grade 3 tumors are more aggressive than grade 2 tumors, the prognosis is still substantially better than for most other grade 3 brain tumors, with median survival often in the range of 10–15 years following standard treatment. The combination of an IDH mutation, 1p/19q codeletion, and standard treatment with chemotherapy and radiation produces some of the best outcomes seen in the treatment of any high-grade glioma.

Biomarker and molecular testing

Molecular testing is an essential part of the workup for every oligodendroglioma. The results confirm the diagnosis (because both an IDH mutation and 1p/19q codeletion are required), distinguish oligodendroglioma from other diffuse gliomas, determine eligibility for targeted therapy, and identify patients who may benefit from clinical trials.

IDH1 and IDH2

Mutations in the IDH1 and IDH2 genes are required for the diagnosis of oligodendroglioma. These genes normally help cells produce energy; when mutated, they produce an abnormal enzyme that generates a molecule called 2-hydroxyglutarate, which disrupts normal cell function and drives tumor growth. Testing is performed in two steps. The first is an immunohistochemistry stain for the most common IDH1 mutation (R132H); a positive result confirms that the tumor is IDH-mutant. The second is DNA sequencing of IDH1 and IDH2, performed when the immunohistochemistry stain is negative, to detect rarer IDH mutations. IDH mutations are not only diagnostic — they also determine eligibility for the targeted therapy vorasidenib, described below.

1p/19q codeletion

Chromosomes are long DNA strands that contain genes, with a short arm (labeled “p”) and a long arm (labeled “q”). In oligodendroglioma, the short arm of chromosome 1 and the long arm of chromosome 19 are both lost — a change called 1p/19q codeletion. This combined loss is the defining genetic feature of oligodendroglioma and is required for the diagnosis. The deletion is detected using fluorescence in situ hybridization (FISH), polymerase chain reaction (PCR), next-generation sequencing, or DNA methylation profiling. The result is reported as 1p/19q codeleted or 1p/19q intact. Tumors that are IDH-mutant but lack the 1p/19q codeletion are classified as astrocytoma, IDH-mutant, not oligodendroglioma — even when the cells look oligodendroglial under the microscope.

TERT promoter mutation

The TERT gene makes a protein that lengthens telomeres, the protective caps on the ends of chromosomes. Mutations in the TERT promoter (a regulatory region that controls whether the gene is turned on) are present in nearly all oligodendrogliomas. Although TERT promoter mutations are not required for diagnosis, they are commonly tested because their presence, in combination with an IDH mutation and 1p/19q codeletion, further supports the diagnosis.

CIC and FUBP1

The CIC gene is located on chromosome 19q, and the FUBP1 gene is located on chromosome 1p — the two regions affected by the 1p/19q codeletion. Mutations in these genes are common in oligodendroglioma, although testing for them is not required for diagnosis. When detected, they provide additional evidence of an oligodendroglial tumor.

Vorasidenib eligibility

The identification of an IDH mutation also determines eligibility for a targeted therapy called vorasidenib (brand name Voranigo). Vorasidenib is a once-daily pill that blocks the abnormal enzyme produced by mutations in the IDH1 and IDH2 genes, reducing 2-hydroxyglutarate production and slowing tumor growth. In August 2024, vorasidenib received approval from the U.S. Food and Drug Administration for adult and pediatric patients 12 years and older with grade 2 astrocytoma or oligodendroglioma with an IDH1 or IDH2 mutation, following surgery. Approval was based on the INDIGO clinical trial, which showed that vorasidenib reduced the risk of tumor progression by about 60% compared to placebo and significantly delayed the need for radiation or chemotherapy. Vorasidenib is not currently approved for grade 3 tumors, and it is not approved for patients who have already received radiation or chemotherapy. Vorasidenib is the first major therapeutic advance for IDH-mutant gliomas in more than two decades and represents a meaningful new option for many patients with grade 2 oligodendroglioma. Approval in Canada and other countries is evolving; ask your treatment team about current access.

DNA methylation profiling

DNA methylation refers to small chemical tags attached to DNA that help control which genes are turned on or off. Different tumor types have distinct methylation patterns, almost like a fingerprint. DNA methylation profiling compares a tumor’s pattern to a large reference database. It is increasingly used in specialized centers to confirm the diagnosis of oligodendroglioma and to distinguish it from other diffuse gliomas. This testing is especially useful when the microscopic findings are ambiguous or when tissue is limited.

For more information about biomarkers and molecular testing across all cancer types, visit the Biomarkers and Genetic Testing section.

What is the prognosis?

Oligodendroglioma has the most favorable prognosis among the diffuse gliomas. The combination of IDH mutation and 1p/19q codeletion identifies a tumor that responds particularly well to treatment, especially to a combination of radiation and chemotherapy. Typical median survival figures are:

• WHO grade 2 oligodendroglioma — median survival is often 15 years or longer, with many patients living substantially longer with modern treatment. Tumors progress slowly, and patients often have many years of stable disease between treatments.
• WHO grade 3 oligodendroglioma — median survival is approximately 10–15 years following standard treatment with surgery, radiation, and chemotherapy — substantially better than for grade 3 IDH-mutant astrocytoma and far better than for IDH-wildtype glioblastoma.

Several features influence the outlook:

• Younger age — patients under 40 generally do better than older patients.
• Good overall health and performance status — the ability to carry out normal daily activities at the time of diagnosis — are among the strongest predictors of survival.
• Complete or near-complete surgical removal — patients whose tumors can be largely or entirely removed generally live longer.
• Frontal lobe location — tumors in the frontal lobes are often easier to remove safely than tumors in deeper or more critical locations.
• Seizures as the presenting symptom — patients whose tumor is identified because of a seizure (rather than because of progressive neurological symptoms) tend to do better, likely because their tumors are diagnosed earlier and at a smaller size.
• Lower WHO grade — grade 2 tumors have substantially longer median survival than grade 3 tumors.

Oligodendrogliomas can come back after treatment, sometimes years or decades later, and a small number eventually transform into more aggressive tumors. Long-term monitoring with regular imaging is essential. Because treatment has changed substantially in recent years — particularly with the introduction of vorasidenib for grade 2 tumors — older survival figures may underestimate how well many patients do today.

What happens after the diagnosis?

Oligodendroglioma is managed by a multidisciplinary team that typically includes a neurosurgeon, a neuro-oncologist, a radiation oncologist, a neuropathologist, and a neuroradiologist. Other members of the team may include a neurologist for seizure management, a neuropsychologist, rehabilitation specialists (physical, occupational, and speech therapy), a social worker, and palliative care, which is often introduced early and alongside active treatment. A geneticist or genetic counselor is involved when an inherited condition is suspected.

Treatment depends on the grade of the tumor, the patient’s age and overall health, and how much of the tumor can be safely removed during surgery.

Grade 2 tumors

For grade 2 tumors, treatment begins with maximal safe surgical removal. After surgery, the next step depends on whether the tumor is considered high-risk (for example, in patients over 40, or when the tumor could not be fully removed) or lower-risk.

• Vorasidenib — since its approval in August 2024, vorasidenib has become an important option for grade 2 tumors following surgery, particularly for patients who do not yet need radiation or chemotherapy. The drug is a once-daily pill taken for as long as it continues to work. It has been shown to delay tumor progression and the need for more intensive therapy. Vorasidenib can be especially valuable for younger patients, who often wish to delay radiation as long as possible because of its potential long-term effects on cognitive function.
• Observation — for some patients with small, completely removed tumors, careful observation with regular MRI scans (every 3–6 months) may be appropriate before starting any medical therapy.
• Radiation combined with chemotherapy — for higher-risk grade 2 tumors, or when the tumor grows or progresses, the standard treatment is radiation followed by PCV chemotherapy (a combination of procarbazine, lomustine, and vincristine). The RTOG 9802 clinical trial showed that adding PCV to radiation substantially improves survival for high-risk low-grade gliomas, with the largest benefit seen in 1p/19q codeleted tumors.

Grade 3 tumors

For grade 3 oligodendroglioma, the standard treatment is maximal safe surgical removal followed by radiation combined with PCV chemotherapy. Two landmark clinical trials (RTOG 9402 and EORTC 26951) showed that adding PCV to radiation therapy dramatically improves survival in grade 3 oligodendroglioma — a benefit limited to tumors with the 1p/19q codeletion. PCV is therefore the preferred chemotherapy for oligodendroglioma based on long-term follow-up data, although temozolomide is sometimes used as an alternative because it has fewer side effects. Vorasidenib is not currently approved for grade 3 tumors but is being studied in clinical trials as a potential addition to or replacement for chemotherapy.

Follow-up and long-term care

Regardless of grade, long-term follow-up is essential. Patients are followed with regular MRI scans to check for tumor growth, with the interval determined by the grade and stability of the tumor. Long-term effects of the tumor and its treatment — on cognitive function, memory, mood, seizure control, hormones, and overall neurological function — are managed by the multidisciplinary team. Neuropsychological testing, rehabilitation therapies, seizure management, and mental health support are important parts of survivorship care. Because oligodendroglioma often occurs in young adults in the prime of their careers and family life, practical support around work, driving, parenting, and family planning is also an important part of the conversation with the treatment team.

Palliative care focuses on comfort, symptom management, and emotional and spiritual support, and is compatible with ongoing cancer-directed treatment. Early palliative care has been shown to improve quality of life for both patients and families and is increasingly introduced as a standard part of care rather than only at the end of life.

Questions to ask your doctor

• Does my tumor have both an IDH mutation and 1p/19q codeletion, confirming the diagnosis of oligodendroglioma?
• What is the WHO grade of my tumor — 2 or 3 — and what does the grade mean for my prognosis?
• How much of the tumor was removed during surgery?
• Am I eligible for vorasidenib? If so, when would I start it,, and what should I expect?
• Is radiation, chemotherapy, or both part of my initial treatment plan, or can I start with vorasidenib or observation?
• If chemotherapy is recommended, do you suggest PCV or temozolomide, and why?
• What clinical trials are available for my grade and molecular profile?
• How often will I need MRI scans, and how will we know if the treatment is working?
• What symptoms should I watch for that might indicate the tumor is growing?
• How will seizures be managed, and will I be able to drive?
• What are the expected long-term effects of the tumor and its treatment on cognitive function, memory, and mood?
• What supportive care services are available — neuropsychology, rehabilitation, mental health, and palliative care?
• Should I be referred for genetic counseling or germline genetic testing?
• How can my family and I plan for the road ahead, including work, parenting, and family planning?

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