Tumors are masses consisting of abnormal tissue that have a tendency to grow more rapidly than normal, and do not respond to mechanisms set into action by the body to control the growth of the mass. Brain tumors arise within the brain tissue (either from nerve cells or cells that supply support to the nerve cells) or from structures within the skull that are immediately adjacent to the brain. Similarly, spinal cord tumors can arise within the substance of the spinal cord or from the tissue immediately adjacent to it. Tumors of the brain and spinal cord are typically classified as benign (slow growing) or malignant (faster growing). In the pediatric population, either type has the potential of spreading to other sites within the nervous system, but malignant tumors are much more likely to do so. Spread outside of the nervous system is very uncommon in either case.
Brain tumors are the most common form of solid tumors in children. The frequency for newly diagnosed brain tumors is about three to four per 100,000 children per year. The brain is subdivided into two major spaces by a thick sheet of tissue called the tentorium. In one compartment (the supratentorial) lie the cerebral hemispheres (the large halves of the brain) and in the other (the posterior fossa) are the cerebellum and brainstem. In young children (less than two years) tumors are slightly more common in the supratentorial, whereas in older children the posterior fossa is a more common site.
Tumors of the spinal cord or adjacent tissue are much less common in children than brain tumors. About half arise outside the spinal sac, usually from the bone or adjacent structures. Forty percent form within the spinal cord, while 10% grow within the spinal sac but outside of the spinal cord itself thus arise from the tissue adjacent to the spinal cord.