A Brain Tumor
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A brain tumor is any intracranial mass created by an abnormal and uncontrolled growth of cells either normally found in the brain itself: neurons, glial cells (astrocytes, oligodendrocytes, ependymal cells), lymphatic tissue, blood vessels), in the cranial nerves (myelin producing Schwann cells), in the brain envelopes (meninges), skull, pituitary and pineal gland, or spread from cancers primarily located in other organs (metastatic tumors).Primary (true) brain tumors are commonly located in the posterior cranial fossa in children and in the anterior two-thirds of the cerebral hemispheres in adults, although they can affect any part of the brain.In the United States in the year 2000, it was estimated that there were 16,500 new cases of brain tumors1, which accounted for 1.4% of all cancers, 2.4% of all cancer deaths2, and 20%?25% of pediatric cancers2,3. Ultimately, it is estimated that there are 13,000 deaths/year as a result of brain tumors. Causes Aside from exposure to vinyl chloride or ionizing radiation, there are no known environmental factors associated with brain tumors. Mutations and deletions of so-called tumor suppressor genes are incriminated in some forms of brain tumors. Patients with various inherited diseases, such as Von Hippel-Lindau syndrome, multiple endocrine neoplasia, neurofibromatosis type 2 are at high risk of developing brain tumors. Classification Primary tumors In contrast to tumors originating elsewhere in the body, differentiating primary "brain tumors"?these are the true brain tumors, arising exclusively from cells normally present in the brain itself?into benign and malignant is of relative and limited clinical value, since even histological-benign tumors grow by infiltration of healthy brain tissue and, in time, tend to transform into malignant forms (anaplastic degeneration). True benign intracranial tumors arise mainly from the meninges (meningiomas; about 95% are benign), pituitary gland (pituitary adenomas) and the myelin sheath of cranial nerves (neuromas or Schwanomas, e.g. acoustic neuroma).Most primary brain tumors (gliomas) originate from glia: astrocytes (astrocytomas), oligodendrocytes (oligodendrogliomas). There are also mixed forms, with both an astrocytic and an oligodendroglial cell component. These are called mixed gliomas or oligoastrocytomas. Additionally, mixed glio-neuronal tumors (tumors displaying a neuronal, as well as a glial component, e.g. gangliogliomas, disembryoplastic neuroepithelial tumors) and tumors originating from neuronal cells (e.g. gangliocytoma, central gangliocytoma) can also be encountered. Other varieties of primary brain tumors include: primitive neuroectodermal tumors (PNET, e.g. medulloblastoma, medulloepithelioma, neuroblastoma, retinoblastoma, ependymoblastoma), tumors of the pineal parenchyma (e.g. pineocytoma, pineoblastoma), ependymal cell tumors, choroids plexus tumors, intraepithelial tumors of uncertain origin (e.g. gliomatosis cerebri, astroblastoma) etc From a histological perspective, astrocytomas, oligondedrogliomas, and oligoastrocytomas may be benign or malignant. Glioblastoma multiforme represents the most aggressive variety of malignant glioma. At the opposite end of the spectrum, there are so-called pilocytic astrocytomas, a distinct variety of astrocytictumors. The majority of them are located in the posterior cranial fossa, affect mainly children and young adults, and have a clinically favorable course and prognosis. In contrast to other types of cancer, primary brain tumors rarely metastasize, and in this rare event, the tumor cells spread within the skull and spinal canal through the cerebrospinal fluid, rather than via bloodstream to other organs. There are various classification systems currently in use for primary brain tumors, the most common being the World Health Organization (WHO) brain tumor classification, introduced in 1993.
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