Brain and spinal cord -
This article was last reviewed and updated on January 30, 2019
by Jane Barron, MD FRCPC
Oligodendroglioma is a type of tumour that can develop in the brain and rarely in the spinal cord.
The tumour can start at any age but most develop in adults.
Your pathology report for oligodendroglioma will include important information such as the IDH and 1p/19q status of the tumour.
The normal brain and spinal cord
The brain is an organ that is found within the skull. It is part of a system known as the central nervous system which includes the brain and spinal cord. The brain is divided into the cerebrum, cerebellum, and brainstem. The spinal cord is a long thin structure that sits in the spinal column at the back of the body. The spinal cord starts in the neck and ends in the lower back.
The brain and spinal cord are made up of specialized cells called glial cells and neurons. The glial cells are further divided into cells called astrocytes, oligodendrocytes, ependymal cells, and microglial.
What is an oligodendroglioma?
Oligodendroglioma is a type of tumour that can start in the brain or spinal cord. It is made of abnormal oligodendrocytes. This tumor can occur at any age but most start in adults.
World Health Organization (WHO) Grade
Pathologists use the word grade to describe how different the tumour cells in oligodendroglioma look compared the cells normally found in the brain or spinal cord. Brain and spinal cord tumors are given a WHO grade of I to IV.
For oligodendroglioma, the grade can only be determined after your pathologist has examined the tissue under the microscope.
Why is this important? According to the WHO, oligodendrogliomas can be given a grade of II or III. Another name for a grade III oligodendroglioma is anaplastic oligodendroglioma.
When viewed under a microscope, WHO grade II oligodendrogliomas have the following features:
Tumour cells – The cells can have uniform round nuclei surrounded by a clear space. Some pathologists call this a honeycomb or fried egg appearance.
Cellularity – The number of cells present in the tumour is moderately increased compared to normal brain tissue.
Nuclear atypia –The nucleus is the part of the cell that contains genetic material or DNA. Cells with nuclear atypia have nuclei that are darker, larger, or more irregular in shape compared to normal oligodendroglial cells. WHO grade II oligodendrogliomas can have some nuclear atypia.
Mitoses - A tumour cell that is in the process of dividing to create new tumour cells is called a mitotic figure. Mitotic activity is a measure of how many tumour cells are dividing to create new tumour cell. A grade II oligodendroglioma should have no mitoses or a low number of mitoses.
Calcifications – Calcium is a mineral that is normally found inside cells. When cells die they release their calcium into the surrounding tissue which stick together to form calcifications. Calcifications can be seen in oligodendrogliomas.
Delicate blood vessels – The blood vessels in the tumour have thin walls and can branch. Some pathologists may call this a chicken-wire appearance.
Cysts – Spaces within the tumour may be seen. This may be described as cystic change. Sometimes these cysts can contain material that pathologists call mucoid material.
When viewed under a microscope, WHO grade III oligodendrogliomas or anaplastic oligodendrogliomas have the following features:
Cellularity- The number of cells in the tumour present is greatly increased (hypercellular) compared to normal brain tissue.
Nuclear atypia -The nucleus is the part of the cell that contains genetic material or DNA. Cells with nuclear atypia have nuclei that are darker, larger, or more irregular in shape compared to normal oligodendroglial cells. WHO grade III oligodendrogliomas will have nuclear atypia.
Microvascular proliferation - Tumours require lots of blood in order to grow. WHO grade III oligodendrogliomas can create new blood vessels and pathologists call this process microvascular proliferation.
Necrosis - Necrosis is a form of cell death. Fast growing tumours such as WHO grade III oligodendrogliomas can often contain many dead tumour cells. As a result, necrosis is very commonly seen in WHO Grade III oligodendrogliomas. Necrosis may also be seen after a tumour has been treated with radiation or chemotherapy.
Mitotic activity - A tumour cell that is in the process of dividing to create new tumour cells is called a mitotic figure. Mitotic activity is a measure of how many tumour cells are dividing to create new tumour cell. A grade III oligodendroglioma will have mitoses.
Why is this important? Grade is important, as WHO grade III oligodendrogliomas can have a worse prognosis compared to a WHO grade II oligodendroglioma.
Isocitrate Dehydrogenase (IDH)
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-mutant.
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.
Your pathologist may perform additional immunohistochemistry tests and oligodendrogliomas may be positive for other proteins such as MAP2, S100, LEU7, vimentin, and GFAP.
1p/19q co-deletion testing
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 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.
Oligodendroglioma, not otherwise specified (NOS)
If the testing for a mutation in IDH and 1p19q co-deletion cannot be performed or the testing is inconclusive, but the tumor has the microscopic features of an oligodendroglioma, your pathologist will make the diagnosis of oligodendroglioma, not otherwise specified (NOS).