Resumo de A Brain Tumor

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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