A meningioma is a tumor that arises from the meninges — the membranes that surround your brain and spinal cord. Although not technically a brain tumor, it is included in this category because it may compress or squeeze the adjacent brain, nerves and vessels. Meningioma is the most common type of tumor that forms in the head. Most meningiomas grow very slowly, often over many years without causing symptoms. But sometimes, their effects on nearby brain tissue, nerves or vessels may cause serious disability. Meningiomas occur more commonly in women and are often discovered at older ages, but may occur at any age. Because most meningiomas grow slowly, often without any significant signs and symptoms, they do not always require immediate treatment and may be monitored over time. A meningioma is a tumor that forms on membranes that cover the brain and spinal cord just inside the skull. Specifically, the tumor forms on the three layers of membranes that are called meninges. These tumors are often slow-growing. As many as 90% are benign (not cancerous). Most meningiomas occur in the brain. But they can also grow on parts of the spinal cord. Often, meningiomas cause no symptoms and require no immediate treatment. But the growth of benign meningiomas can cause serious problems. In some cases, such growth can be fatal. Meningiomas are the most common type of tumor that originates in the central nervous system. They occur more often in women than in men. Some meningiomas are classified as atypical. These are not considered either benign or malignant (cancerous). But they may become malignant. A small number of meningiomas are cancerous. They tend to grow quickly. They also can spread to other parts of the brain and beyond, often to the lungs.