Tumor Types > Oligodendrogliomas
Oligodendrogliomas comprise a class of glial tumors in which the oligodendroglial cell is the predominant cell type. Normally, oligodendroglial cells form myelin - the fatty substance which surrounds the axons of nerve cells, providing insulation so that nerve cell electrical transmission is faster and more efficient. Oligodendrogliomas probably evolve from a mixed glioma, which has in turn evolved from primitive precursor or stem cells. The mixed glioma is a slow growing tumor, containing astrocytic and neuronal elements as well as oligodendroglial cells.
Each of these cell types have a percentage of cells capable of mitosis (the process by which one cell divides to become two cells). In oligodendrogliomas, the mitotic rate of oligodendroglial cells exceeded the rate of the other cells. Eventually, the oligodendroglial cells are the most numerous - or "predominant" - cell type, resulting in an oligodendroglioma.
Oligodendrogliomas should not be termed benign or malignant as no glial tumor is so prognostically black or white. Instead, oligodendrogliomas are often classified as low grade, anaplastic and malignant types. But these are not concrete definitions either, since low grade tumors can become anaplastic and anaplastic tumors can become malignant. Because the speed of this transformation varies widely from patient to patient, there is no consistent way of predicting the behavior of any oligodendroglioma - particularly in adult patients.
In general, all glial tumors - including oligodendrogliomas - will become malignant. It is just a matter of time and random evolution for the cells within the tumor to increase their rate of mitosis. Cells capable of mitosis will produce offspring with the same mitotic ability. Given enough time, the percentage of cells capable of mitosis will invariably increase, changing the tumor's classification from low grade to anaplastic, and from anaplastic to malignant. How much time does it take for the transformation to occur? It could be as long as 30 years -- or it could be as brief as 6 months.
Tumor Growth Important to know: rate of tumor growth, rate of invasion and replacement of brain tissue by tumor, estimated life expectancy of patient.
Tumor growth depends on the rate of mitosis (birth of new cells) and the rate of apoptosis (cell death). A tumor in which the rate of mitosis is equal to the rate of apoptosis does not appear to grow. It stays the same size, with new tumor cells simply replacing cells as they die, while the number of tumor cells remains the same.
When the mitotic rate is greater than the apoptotic rate, the tumor grows both in the number of cells and in size. Assuming that the apoptotic rate stays the same, the rate of tumor growth will be directly related to the proportion of cells capable of - and undergoing - mitoses. More mitoses means faster growth. In addition, assuming the same rate of mitoses (the percentage of cells undergoing mitosis over the total number of cells in a tumor), a larger tumor will grow faster than a smaller tumor. This is because a larger tumor has more cells, thus more cells undergoing mitosis than in a smaller tumor.
How do we judge growth in an oligodendroglioma? Three ways: progressive enlargement on CT or MRI, development of new symptoms and development of new contrast enhancement. Oligodendrogliomas, like most glial tumors, have two growth patterns: solid tumor tissue and infiltrating tumor cells.
Low Grade Oligodendroglioma
Anaplastic Oligodendroglioma
Malignant Oligodendroglioma
1. Solid tumor tissue This is a mass of solid tumor cells that enlarges as the number of tumor cells within the mass increases. Accompanying the enlargement of the mass is the production of new blood vessels from the blood vessels which supply the brain. These vessels supply the growing tumor cells with oxygen, glucose and other vital necessities. In less malignant tumors, this blood supply may be significantly less prominent than in malignant oligodendrogliomas. This is the reason that a solid tumor tissue mass often does not show contrast enhancement on CT or MRI in a low grade oligodendroglioma. Tumor tissue in an anaplastic or malignant oligodendroglioma requires a rich blood supply, providing prominent contrast enhancement on CT and MRI. From a practical point of view, tumor tissue in a low grade, anaplastic or malignant oligodendroglioma can be removed without neurological deficit (provided that the surgeon can get to it safely). Solid tumor tissue either displaces or replaces brain substance. Removing the tumor tissue does not mean removing functioning brain, as it does is in the second growth pattern, described in the next section.
2. Infiltrating tumor cells Isolated tumor cells are capable of movement within the background brain tissue. They can live quite well between the fibers which make up the brain, deriving nutrients and oxygen from the fluid in which all cells float. These cells can travel a great distance from the main tumor mass. In many oligodendrogliomas, the entire tumor is made up of only isolated tumor cells which reside in "sick" but functional brain tissue. The problem is, there is no good way to remove the tumor cells without damaging the brain tissue in which they reside. Removal of the tumor requires removing intact, functional, (albeit "sick") brain tissue. If this brain tissue is in an important location, the patient would suffer neurological deficits (paralysis, sensory disturbances, speech problems, etc.) following surgery. However, infiltrating tumors can be removed from unimportant brain areas such as the frontal or tip of the temporal lobe. When there is question, some form of mapping procedure is used to determine whether or not the brain tissue is functional before removing it.
Most low grade oligodendrogliomas are comprised of infiltrating tumor cells within intact brain tissue. They are visible as hypodensity on CT and increased T1 and T2 signal on MRI because of the tendency of tumor cells to draw water into the infiltrated brain tissue. All of our imaging studies (CT, MRI) are sensitive to changes in water concentration in comparison to normal levels. However, if isolated tumor cells are not great enough in number to change the local osmolality (and thus increase the amount of water surrounding them), the CT and MRI will show no changes and appear normal in that area. The practical point here is that even if the surgeon is able to remove all of the MRI defined abnormality in a patient with an oligodendroglioma, all of the tumor cells still would not have been removed. And the tumor would eventually come back.