Brain Tumors
The central nervous system is constituted by the brain and the spinal cord. The brain consists of the cerebral hemispheres, the diencephalon, the cerebellum, and the brain stem (Figure .1). Brain and spinal cord tumors occur most often in children younger than 10 years. Brain tumors are characterized by the type of cell and region of the brain from which they originate. Some brain tumors originate in other parts of the body and metastasize to the brain. They are called metastatic brain tumors or secondary brain tumors to distinguish them from the primary brain tumors that originate in the brain.
Neurons are the most important cells (Figure .2) in the brain. Tumors arising from the neurons of the cerebellum are called medulloblastomas. There are four types of glial cells: astrocytes, oligodendrocytes, microglia, and ependymal cells. Tumors arising in glial cells are called gliomas, tumors arising in the astrocytes are called astrocytomas (high-grade astrocytoma is referred to as glioblastoma), tumors arising in the oligodendrocytes are called oligodendrogliomas, and tumors arising in the ependymal cells are called ependymomas.
In children, the most common brain tumor is supratentorial astrocytoma, accounting for 25-40% of all tumors (American Cancer Association, 2004). Supratentorial astrocytomas arise in astrocyte cells outside the cerebellum, brain stem, or spinal cord. The cerebellar astrocytomas account for 10-20% of all cases of brain tumors in children. The brain stem gliomas account for 10-20% of all childhood brain tumors. The medulloblastoma likewise accounts for about 10-20% of brain tumors. More rare forms of childhood brain tumors are ependymoma (5-10%), craniopharyngioma (6-9%), and pineal tumors (0.5-2%).
Diagnosis of brain tumors is typically by magnetic resonance imaging (MRI) and computer tomography (CT) scan, followed by a biopsy of a suspected tumor.
There is no generally accepted staging system for brain tumors. They are, however, graded into low, intermediate, and high grade based on the appearance of cells from Cancer Diagnostics with DNA Microarrays, By