by Jane Barron, MD FRCPC
November 25, 2023
Oligodendroglioma is a type of tumour that can start in the brain or spinal cord. It is made of abnormal oligodendrocytes.
A biopsy is a surgical procedure that removes a small piece of tissue for examination by a pathologist. The purpose of a biopsy is to establish a diagnosis. A test called immunohistochemistry may be performed to confirm the diagnosis. Test results for isocitrate dehydrogenase (IDH) and 1p/19q co-deletion may also be included in your biopsy report. Your doctors will use this information to plan treatment such as surgery and radiation therapy.
After the tumour has been removed completely, it will be sent to a pathologist who will prepare another pathology report. This report will confirm or revise the original diagnosis. If additional immunohistochemical or molecular tests are performed, the results will be described in this report.
Pathologists use the word grade to describe how different the tumour cells in oligodendroglioma look compared to the cells normally found in the brain or spinal cord. The grade can only be determined after a sample of tumour is examined under the microscope by a pathologist. Most brain and spinal cord tumours are given a WHO grade of I to IV. However, in this grading system, oligodendrogliomas can only be given a grade of II or III.
When viewed under a microscope, WHO grade II oligodendrogliomas have the following features:
Another name for a grade III oligodendroglioma is anaplastic oligodendroglioma. When viewed under a microscope, WHO grade III oligodendrogliomas or anaplastic oligodendrogliomas have the following features:
The grade is important, as WHO grade III oligodendrogliomas can have a worse prognosis compared to a WHO grade II oligodendroglioma.
Your pathologist may perform a test called immunohistochemistry which allows your pathologist to use specific markers to see the proteins being made by the tumour cells. If the tumour cells are making a protein, the result will be described as ‘positive’ or ‘reactive’.
There are no specific immunohistochemical tests for oligodendroglioma cells. However, most oligodendrogliomas will produce a protein called isocitrate dehydrogenase (IDH) and will test positive when immunohistochemistry for IDH is performed.
There are two forms of the IDH protein made by the genes IDH1 and IDH2. A genetic alteration, called a mutation, can occur in either of these genes which causes the cell to produce an abnormal IDH protein. Oligodendrogliomas that have a genetic alteration, a mutation, in IDH1 or IDH2, are called IDH-mutants.
There is an antibody available for IDH1 and most oligodendrogliomas will test positive for the IDH1 antibody but if this test is negative it does not rule out the possibility of a mutation. Further molecular testing, called DNA sequence analysis, will need to be performed to be certain if an IDH 1 or IDH 2 mutation is present.
Each cell in your body contains a set of instructions that tell the cell how to behave. These instructions are written in a language called DNA and the instructions are stored on 46 chromosomes in each cell. Because the instructions are very long, they are broken up into sections called genes and each gene tells the cell how to produce a piece of the machine called a protein.
Sometimes, a piece of DNA falls off a chromosome and is lost. Pathologists call this a deletion. When a deletion occurs, the cell can no longer produce proteins made from the genes on the lost piece of DNA.
Oligodendrogliomas have a deletion that involves two chromosomes. These chromosomes are chromosomes 1 and 19. Your pathologist can look for this deletion by using a test called fluorescence in situ hybridization (FISH). If the deletion is found, your report will say that the tumour shows a 1p19q co-deletion.
Your pathologist may perform additional immunohistochemistry tests and oligodendrogliomas may be positive for other proteins such as MAP2, S100, LEU7, vimentin, and GFAP.
If the testing for a mutation in IDH and 1p19q co-deletion cannot be performed or the testing is inconclusive, but the tumour has the microscopic features of an oligodendroglioma, your pathologist will make the diagnosis of oligodendroglioma, not otherwise specified (NOS).
This article was written by doctors to help you read and understand your pathology report. Contact us if you have any questions about this article or your pathology report. Read this article for a more general introduction to the parts of a typical pathology report.