Types of central nervous system tumors
There are many different types of brain tumors, based on what cells are affected and how they appear under a microscope. Tumors can be classified into four general categories: gliomas, meningiomas, acoustic neuroma and medulloblastoma.
These tumors occur in the glial cells, which help support and protect critical areas of the brain. Gliomas are the most common type of brain tumor in adults, responsible for about 42 percent of all adult brain tumors. Brain stem gliomas that are high-grade or spread widely throughout the brain stem are difficult to treat successfully. To prevent damage to healthy brain tissue, brain stem glioma is usually diagnosed without a biopsy.
Gliomas are further characterized by the types of cells they affect:
- Astrocytes: These are star-shaped cells that protect neurons. Tumors of these cells can spread from the primary site to other areas of the brain, but they rarely spread outside the central nervous system. Astrocytomas are graded from I to IV depending on the speed of progression:
- Grade I (pilocytic astrocytoma): Slow growing, with little tendency to infiltrate surrounding brain tissue. Most common in children and adolescents.
- Grade II (diffuse astrocytoma): Fairly slow-growing, with some tendency to infiltrate surrounding brain tissue. Mostly seen in young adults.
- Grade III (anaplastic/malignant astrocytoma): These tumors grow rather quickly and infiltrate surrounding brain tissue.
- Grade IV (glioblastoma multiforme, GBM): An extremely aggressive and lethal form of brain cancer. Unfortunately, it is the most common form of brain tumor in adults, accounting for 67 percent of all astrocytomas.
- Oligodendroglioma: Oligodendrocytes are cells that make myelin, a fatty substance that forms a protective sheath around nerve cells. Oligodendrogliomas, which make up 4 percent of brain tumors, mostly affect people age 45 and older. Some subtypes of this tumor are particularly sensitive to treatment with radiation therapy and chemotherapy. Half of patients with oligodendrogliomas are still alive after five years.
- Ependymoma: These tumors affect ependymal cells, which line the pathways that carry cerebrospinal fluid throughout the brain and spinal cord. Ependymomas are rare. They make up about 2 percent of all brain tumors, but they are the most common brain tumor in children. They generally don't affect healthy brain tissue and don't spread beyond the ependyma. Although these tumors respond well to surgery, particularly those on the spine, ependymomas cannot always be completely removed. The five-year survival rate for patients over age 45 approaches 70 percent.
These tumors affect the meninges, the tissue that forms the protective outer covering of the brain and spine. One-quarter of all brain and spinal tumors are meningiomas, and up to 85 percent of them are benign. Meningiomas can occur at any age, but the incidence increases significantly in people over age 65. Women are twice as likely as men to have meningiomas. They generally grow very slowly and often don't produce any symptoms. In fact, many meningiomas are discovered by accident. Meningiomas can be successfully treated with surgery, but some patients, particularly the elderly, may be candidates for watchful waiting to monitor the disease.
Schwann cells are found in the sheath that covers nerve cells. Vestibular schwannomas, also known as acoustic neuromas, arise from the eighth cranial nerve, which is responsible for hearing. Specific symptoms of vestibular schwannoma include buzzing or ringing in the ears, one-sided hearing loss and/or balance problems. Schwannomas are typically benign and respond well to surgery.
Medulloblastoma is a common brain tumor in children, usually diagnosed before the age of 10. These tumors occur in the cerebellum, which has a crucial role in coordinating muscular movements. Some experts believe that medulloblastomas arise from fetal cells that remain in the cerebellum after birth. Tumors grow quickly and can invade neighboring portions of the brain, as well as spreading outside the central nervous system. Medulloblastoma is slightly more common in boys.