A brain tumor is a mass created by an 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 adulthood.

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 are usually, but not necessarily, localized to a small area. They can also be malignant and invasive (i.e., 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 sutures.

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.