The most common type of primary brain tumor is the meningioma, a noncancerous tumor that arises from the abnormal growth of the cells that form the delicate outer covering of the brain, called the meninges.
Approximately 96% of meningiomas occur within the skull, with the remaining 4% involving the spinal column. Meningiomas are the most common type of primary brain tumor, accounting for 34.4% of all such tumors. A primary brain tumor originates from the brain, spinal cord or associated tissues (called the central nervous system or CNS), while a secondary brain tumor arises from cancerous cells that have spread (metastasized) to the CNS from elsewhere in the body.
Most meningiomas (90%) are categorized as benign tumors, approximately 7–8% as atypical, and 2–3% as malignant. See the Types of Meningiomas section for a fuller discussion of these grades of meningioma. In general, most meningiomas do not actually invade brain tissue, although those that are malignant are likely to do so. Meningiomas affect the CNS by compressing brain tissue, nerves, or associated structures; causing reactive swelling in brain tissue; blocking flow of the fluid (cerebrospinal fluid) in the CNS; and/or blocking blood flow in CNS blood vessels. Each of these effects may result in various disabilities or even be life-threatening. Clearly, the term "benign" can be quite misleading in that such meningiomas may cause significant harm. However, benign meningiomas, as compared to those that are atypical and malignant, are more likely to exhibit slower growth, are less likely to reoccur, and rarely invade brain tissue.
Because most meningiomas often grow slowly, depending upon their location, a meningioma may reach a relatively large size before it causes symptoms. There is no way to know for certain how long a specific tumor was growing before diagnosis. Most people with a meningioma will have a tumor at only one site, but it is also possible to have several tumors growing simultaneously in different parts of the brain and spinal cord.