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Intracranial and spinal neoplasms may be primary or metastatic; in most autopsy series, metastatic tumors are more common. Primary intracranial neoplasms number about 13,000 new cases per year in the United States and represent about 2% of deaths from malignant neoplasms. They are the second most common group of neoplasms in children, after leukemia and lymphoma if considered as one group. Taken overall, 65% of primary intracranial neoplasms are of glial origin (gliomas), 10% meningiomas, 10% acoustic schwannomas, 5% medulloblastomas, and 10% others. Primary malignant lymphomas of the central nervous system have recently increased in frequency because they are common in patients with acquired immunodeficiency disease (AIDS). Tumors of neurons per se are extremely uncommon except in childhood (eg, medulloblastoma).
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Histogenetic Classification
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Classification on a histogenetic basis has great theoretical value and provides a means of logically remembering all the different kinds of intracranial neoplasms (Table 65-1).
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Topographic Classification
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When a patient presents with an intracranial neoplasm, its location can usually be ascertained by clinical examination and radiologic studies. According to their location, intracranial neoplasms may be classified as supratentorial or infratentorial. Further subdivisions in these main compartments are recognized (Table 65-2), leading to a topographic classification. When the location of the neoplasm is combined with the patient's age, a clinically useful differential diagnosis of the histologic type of the neoplasm can be derived. For example, if a child presents with a neoplasm in a cerebellar hemisphere, it is most likely a juvenile pilocytic astrocytoma (Table 65-2).
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Classification According to Biologic Potential
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The criteria used to determine malignancy in neoplasms are somewhat different from those used elsewhere in the body:
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Even highly malignant intracranial neoplasms generally do not metastasize outside the craniospinal axis (Fig 65-1). Metastasis within the craniospinal axis via the cerebrospinal fluid does occur, most commonly with medulloblastoma, pineoblastoma, malignant ependymoma, pineal germinoma, and glioblastoma multiforme.
Destructive infiltration of the brain is the major criterion of malignancy for intracranial neoplasms, and infiltration of brain substance usually prevents complete removal at surgery. All glial neoplasms invade brain, and all must be considered malignant. Neurologic deficits resulting from destructive invasion by malignant neoplasms are irreversible. Benign neoplasms, on the other hand, cause neurologic deficits due to compression; these often reverse when the neoplasm is removed.
The rate of growth of neoplasms also correlates well with malignant behavior. Rapidly growing neoplasms such as glioblastoma multiforme and medulloblastoma are highly malignant. Low-grade malignant neoplasms such as well-differentiated astrocytoma and oligodendroglioma grow slowly. Benign neoplasms usually grow very slowly, enlarging over several years.
Recurrence after treatment is almost invariable with malignant intracranial neoplasms. Recurrence also occurs with many benign neoplasms such as meningioma and craniopharyngioma, and therefore recurrence of itself is not a criterion of malignancy.
The term benign for any intracranial neoplasm is probably inappropriate. Benign intracranial neoplasms frequently produce extremely serious clinical disease and may cause severe neurologic deficits and death unless treated. The term thus does not mean that these neoplasms are harmless but implies rather that they are slow growing and do not infiltrate the brain substance.
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Pathology & Clinical Features
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The specific clinicopathologic features of intracranial neoplasms will be considered with the individual neoplasms. In general, intracranial neoplasms cause the following clinical and pathologic changes:
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Compression of adjacent neural tissues occurs with all expanding neoplasms. When the rate of growth is slow, compression leads to atrophy, which may cause symptoms of dysfunction—eg, atrophy of the motor cortex adjacent to a meningioma causes upper motor neuron paralysis; compression of a cranial nerve may cause cranial nerve palsy. In general, relief of compression is followed by significant recovery of function. With long-standing compression, there may be a permanent deficit.
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Destruction of neural tissues by direct infiltration with a malignant neoplasm produces an irreversible deficit.
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Cerebral edema is commonly present around infiltrative neoplasms and may be severe. It is believed to result from the neovascularization that accompanies malignant neoplasms. The new vessels have a poorly developed blood–brain barrier that permits exit of proteins and fluids more easily than from normal vessels. Cerebral edema tends to be most marked in highly malignant neoplasms. Cerebral edema causes elevation of intracranial pressure that is additive to the mass effect of the tumor.
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Irritation of neural tissues may occur with both compressing and infiltrating neoplasms. Abnormal stimulation is usually manifested as either simple or complex partial focal epilepsy. A neoplasm near the motor cortex may generate an abnormal electrical potential that causes motor stimulation of the entire contralateral half of the body (jacksonian epilepsy). Up to 5% of individuals with intracranial neoplasms experience one or more seizures. Note that although only a minority of cases of epilepsy are due to tumors, seizures should be fully investigated for cause due to tumor or other treatable disease, particularly when the onset is in adult life or when the seizure is focal rather than generalized (see Chapter 62: The Central Nervous System: I. Structure & Function; Congenital Diseases).
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Neoplasms in the region of the third ventricle or in the posterior fossa may cause obstructive hydrocephalus. This causes marked elevation of intracranial pressure.
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Increased Intracranial Pressure
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Intracranial neoplasms cause increased intracranial pressure due to (1) the mass effect of the neoplasm itself, (2) cerebral edema, or (3) hydrocephalus. Many patients with intracranial neoplasms present with the effects of increased intracranial pressure—headache, vomiting, papilledema, and false localizing signs due to displacement of the brain and to herniations (see Chapter 62: The Central Nervous System: I. Structure & Function; Congenital Diseases). Shift of structures can often be detected radiographically and provides a clue to the site of intracranial neoplasms.
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Clinical examination and radiologic imaging provide excellent localization of mass lesions of the nervous system. Specific diagnosis, however, is based on microscopic examination of a sample from the tumor. Stereotactic biopsy and open resection are the methods available for obtaining tissue samples. Examination of cerebrospinal fluid is rarely useful because (1) lumbar puncture is not usually performed in the presence of mass lesions, and (2) the yield of neoplastic cells in cerebrospinal fluid is low even with highly malignant neoplasms.