Your pathology report for oligodendroglioma

By Jason Wasserman MD PhD FRCPC
July 15, 2025

Oligodendroglioma is a type of brain tumour that starts in glial cells, the supporting cells of the brain. It is considered a diffuse glioma, meaning the tumour tends to spread into the surrounding brain tissue. Two specific genetic features define this type of tumour:

  1. A mutation in the IDH1 or IDH2 gene.

  2. Loss (codeletion) of two parts of DNA, called chromosome arms 1p and 19q.

These genetic changes are important because they help doctors diagnose the tumour accurately and predict how it will behave. Oligodendrogliomas are classified as either grade 2 (lower grade, slower growing) or grade 3 (higher grade, faster growing) based on how the tumour looks under the microscope and other features like the number of dividing cells.

Is oligodendroglioma a type of brain cancer?

Yes, oligodendroglioma is considered a type of brain cancer, but it tends to grow slowly and often responds well to treatment. Unlike more aggressive brain cancers like glioblastoma, oligodendrogliomas typically have a better prognosis, especially when they carry both the IDH mutation and the 1p/19q codeletion. These tumours can come back over time and sometimes transform into more aggressive forms, so ongoing monitoring and treatment may be needed.

What are the symptoms of oligodendroglioma?

The symptoms of oligodendroglioma depend on where the tumour is located in the brain and how quickly it is growing.

Common symptoms include:

  • Seizures (the most frequent presenting symptom).

  • Headaches.

  • Weakness or numbness on one side of the body.

  • Changes in speech, vision, or coordination.

  • Cognitive or memory problems.

In some cases, the tumour is found incidentally during imaging done for other reasons.

Where in the brain are oligodendrogliomas found?

Most oligodendrogliomas occur in the frontal lobes of the brain, which control thinking, movement, and behavior. Less commonly, they are found in the temporal, parietal, or occipital lobes. Rarely, they may be found in deeper areas of the brain or the spinal cord. In advanced cases, the tumour may spread along the surface of the brain (called leptomeningeal spread).

What causes oligodendroglioma?

The exact cause of oligodendroglioma is not known. Most cases occur sporadically, meaning they are not inherited. Some rare genetic syndromes have been linked to brain tumours, but oligodendroglioma is rarely seen in these conditions. Certain genetic changes, such as mutations in IDH1, IDH2, and TERT, are almost always present in these tumours and are thought to play a role in tumour development.

What does IDH-mutant mean?

IDH-mutant means that the tumour has a mutation (change) in a gene called IDH1 or IDH2. These genes normally help cells manage energy and metabolism. When they are mutated, they produce an abnormal substance called 2-hydroxyglutarate, which interferes with how cells grow and function.

This mutation is very important in brain tumours because:

  • It is almost always found in lower-grade gliomas like oligodendrogliomas and astrocytomas.

  • It is associated with better response to treatment and longer survival.

  • It helps pathologists distinguish between different types of gliomas.

In oligodendroglioma, the most common mutation is IDH1 p.R132H, and it is usually detected with a special test called immunohistochemistry.

What does 1p/19q-codeleted mean?

1p/19q-codeleted means that the tumour cells are missing parts of two chromosomes—the short arm of chromosome 1 (called 1p) and the long arm of chromosome 19 (called 19q). This combined loss is called a codeletion and is one of the defining features of oligodendroglioma.

Why this matters:

  • The 1p/19q codeletion is required for the diagnosis of oligodendroglioma.

  • It is associated with a better prognosis and stronger response to chemotherapy and radiation.

  • It helps doctors distinguish oligodendroglioma from other types of glioma, such as astrocytoma, which usually do not have this genetic change.

The codeletion is detected using genetic testing methods such as FISH (fluorescence in situ hybridization), PCR, or DNA sequencing.

How is oligodendroglioma diagnosed?

Diagnosis usually involves a combination of imaging, surgery, and genetic testing:

  • MRI or CT scan is used to identify the tumour and guide further evaluation.

  • A biopsy or surgical resection is performed to obtain tissue.

  • A pathologist examines the tissue under a microscope and performs genetic tests to check for mutations in the IDH1 or IDH2 gene and codeletion of the chromosome arms 1p and 19q.

Only tumours that have both the IDH mutation and 1p/19q codeletion are officially diagnosed as oligodendroglioma, IDH-mutant and 1p/19q-codeleted.

What does the tumour look like under the microscope?

Under the microscope, oligodendrogliomas typically show the following features:

  • Tumour cells with round nuclei and clear cytoplasm, creating a “fried egg” appearance.

  • A network of thin branching blood vessels that resemble chicken wire.

  • Small areas of calcification (calcium deposits), especially in grade 2 tumours.

  • More crowded cells and more mitotic figures (dividing cells) in grade 3 tumours.

  • A low-grade (grade 2) tumour will have fewer mitotic figures (dividing cells) and no areas of necrosis (dead tumour tissue).

  • A high-grade (grade 3) tumour may show microvascular proliferation (abnormal blood vessels) and necrosis (dead tumour tissue).

What are the grades of oligodendroglioma?

Oligodendrogliomas are classified as:

  • CNS WHO grade 2: These tumours grow slowly and usually do not enhance with contrast on MRI. They have fewer mitotic figures (dividing cells) and no aggressive features under the microscope.

  • CNS WHO grade 3: These tumours grow more quickly, often enhance with contrast, and show more mitotic figures (dividing cells), abnormal blood vessels, or areas of necrosis (dead tumour tissue). They have a higher risk of recurrence and may require more intensive treatment.

The grade helps doctors decide on the best treatment and understand how the tumour is likely to behave over time.

How is oligodendroglioma treated?

Treatment depends on the size, location, grade, and symptoms, as well as how much of the tumour can be removed.

Treatment options may include:

  • Surgery to remove as much tumour as possible.

  • Radiation therapy, often used after surgery for grade 3 tumours or large grade 2 tumours.

  • Chemotherapy, such as a combination called PCV (procarbazine, lomustine, and vincristine) or temozolomide.

Patients with grade 2 tumours and no symptoms may be monitored after surgery, especially if the tumour was fully removed.

What is the prognosis for oligodendroglioma?

Oligodendrogliomas with IDH mutation and 1p/19q codeletion generally have an excellent prognosis, especially compared to other types of gliomas. Many patients live 10 years or more after diagnosis, and some even longer with proper treatment.

Factors linked to better outcomes include:

  • Younger age.

  • Frontal lobe location.

  • Seizures as the first symptom.

  • Complete tumour removal.

  • Low-grade tumour.

The tumour may eventually come back (recur), and in some cases, it can become more aggressive over time. Regular follow-up with imaging is important.

Questions to ask your doctor

  • What grade is my tumour?

  • Was the tumour fully removed?

  • What genetic changes were found?

  • Will I need radiation or chemotherapy?

  • How often will I need follow-up scans?

  • What symptoms should I watch for?

  • Is there a risk the tumour will come back or become more aggressive?

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