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A brain tumor is a mass created by abnormal and uncontrolled growth of cells
either found in the brain (neurons, glial cells, epithelial cells, myelin
producing cells etc.) (primary brain tumors) or originating in another part
of the body and spreading to the brain (secondary brain tumors or metastatic
brain tumors). Brain tumors are usually located in the posterior third of
the brain in childhood and in the anterior two-thirds of the brain in
Primary brain tumors are named due to the cell types, from which they are
originated. Frequently encountered histologic brain tumor types are glioma,
glioblastoma, astrocytoma, oligodendroglioma, medulloblastoma, meningioma
and neuroglioma. Tumors can be benign and usually but not necessarily
localized to a small area, or malignant and invasive (spreading to
neighbouring areas). Brain cells can be damaged by tumor cells by (i)
directly being compressed from growth of the tumor, (ii) indirectly being
affected from inflammation ongoing in and around the tumor mass, (iii) brain
edema (swelling) or (iv) increased pressure in the skull (due to brain edema
or to the blockage of the circulation of the cerebrospinal fluid).
Local tissue damage (either by direct or indirect mechanisms) causes focal
neurologic symptoms, which vary due to the location of the brain tumor.
Hemiparesis, aphasia, difficulty speaking, ataxia, hemihypoesthesia
(numbness and decreased sensation of touch on one side of the body) and
localized headache are some of the symptoms occurring due to the local
effects of the brain tumor. Increased pressure in the skull or brain edema
cause more generalized symptoms like generalized headache, nausea and
vomiting, loss of consciousness (stupor or coma) and intellectual decline.
Seizures due to the local irritating effect of the brain tumor or metabolic
changes caused by the cancer are also frequently observed. Since the
development of the skull is incomplete during infancy, infants with brain
tumor may have increased head perimeter, bulging fontanelles or separated
Neurologic examination reveals local (specific to the location of the tumor)
or generalized neurologic changes. Slowly progressive nature of the
neurologic symptoms is suggestive of a possible brain tumor and the
diagnosis is confirmed by CT scan or MRI of the head. Angiography, EEG
examination or brain biopsy may aid in diagnosis in difficult cases.
Although slow progression is an important hallmark of the disease, some
brain tumors may enlarge very quickly and thus may cause sudden neurologic
changes. Treatment includes the surgical removal of the tumor mass or the
destruction of the tumor cells by radiation (radiotherapy) and/or drugs
(chemotherapy) in cases with contraindications for a surgical operation.
Secondary or metastatic brain tumors take their origins from tumor cells
which spread to the brain from another location in the body. They are more
frequent than primary brain tumors. Approximately, one quarter of metastatic
cancers spread to brain. Lungs and breasts are most common locations from
which secondary brain tumors originate. Tumor cells travel to brain by blood
vessels. Since brain has no lymphatic drainage system like other organs
(cerebrospinal fluid system acts like lymphatic system in the brain),
spreading of tumor cells by lymphatic route (which is very typical for
cancers of other organs) is impossible for brain. Different from primary
brain tumors, metastatic tumor masses may occur in various remote locations
in the brain. Highly aggressive brain tumors like glioblastoma may also be
observed in more than one location but usually in the advanced stages of the
disease. Symptoms, diagnosis and treatment are quite similar to those of
primary tumors, however in case of secondary tumors the initial location of
the tumor cells must be identified and treated, as well.
Primary or secondary, brain tumors may cause herniation of the brain
(displacement of one part of the brain tissue due to mass effect of a
lesion, usually causing the compression of the neurons controlling the
respiratory system in the brainstem and eventually death) and permanent
neurologic changes including intellectual decline.
Tumors located in distant locations may affect the nerve cells and cause
neurologic changes by mechanisms other than directly invading the brain
tissue. Diseases caused by remote effects of tumor cells are called
paraneoplastic diseases. Tumors may affect brain cells from a distance by
consuming too much food and energy that is crucial for neurons, by secreting
endocrine substances altering nerve cell functions or in the majority of the
cases by causing the immune system of the body to develop antibodies
(autoantibodies) directed against nerve cells. In the last mentioned
mechanism, antibodies developed to kill tumor cells are suggested to
accidentally (probably due to molecular similarities between tumor cells and
normal nerve cells) bind neurons and destroy them. Paraneoplastic diseases
due to autoantibodies are not confined to brain cells (e.g. Lambert-Eaton
myasthenic syndrome). Most frequent paraneoplastic diseases are cerebellar
ataxia, peripheral sensory neuropathy, limbic encephalitis and brainstem
encephalitis. The neuroimaging studies are usually not helpful in
paraneoplastic diseases and diagnosis is established by immunological methods.