Cerebral angiography seldom is used in the diagnosis of brain tumors. In a few circumstances, neurosurgeons, in preparation for surgery, require a more precise knowledge of the pattern and position of blood vessels, which can be obtained only by angiography. The procedure is also used to embolize highly vascular meningiomas or to study cerebral dominance by injection of barbiturate into the carotid artery (the Wada test) in left-handed individuals who are to have surgery near language areas. Preoperative determination of cerebral localization helps surgeons to plan the extent of surgery and to avoid creation of postoperative language deficits in the patient.

Examination of the spinal fluid has limited indication in the diagnosis of brain tumors. One is to rule out an inflammatory disorder mimicking a brain tumor. Another is to establish the diagnosis of benign intracranial hypertension in patients with uninformative MRIs. In addition, spinal fluid cytology may be useful for determining instances of malignant meningitis secondary to metastatic neoplasms in association with spinal spread of medulloblastoma in some children and in identifying primary lymphomas of the brain in cases in which MRI changes are ambiguous.

The routine electroencephalogram (EEG) has no role in the diagnosis of brain tumors and does not assist in the choice of anticonvulsant drugs for patients with brain tumor. However, specialized intraoperative neurophysiologic techniques, such as depth electrode studies and intraoperative monitoring, may be useful in identifying and removing epileptogenic areas adjacent to brain tumors or to avoid resection of critical brain regions adjacent to tumors.

Positron emission tomography (PET) is able to quantify biochemical functions, such as oxygen and glucose utilization, within tumors as well as in normal brain tissue. PET scanning is a powerful research tool of limited availability for routine clinical purposes. Its spatial resolution is inferior to that of both CT and MR. In patients with brain tumors who develop recurrent symptoms after radiation therapy, PET can differentiate, with about 70% accuracy, radiation-induced injury from tumor recurrences. These disorders often appear identical on MRI.

Brain imaging by MRI or CT scans is an indispensable component of the modern diagnosis of the presence, but not the type, of brain tumors. One type of tumor can look like another or even resemble a non-neoplastic mass lesion, such as a brain abscess, fungal infection, parasitic invasion, demyelinating disease, or stroke. For definitive diagnosis and adequate treatment planning, one must obtain a tissue diagnosis whenever possible. This can be made either by direct surgical biopsy or, in the case of some non-neoplastic conditions, by judging CT or MRI responses to particular therapies.
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MRI is almost always superior to CT scanning in diagnosing intracranial mass lesions. MRI outlines posterior fossa structures and tumors with a clarity that CT cannot achieve because of x-ray distortions caused by the bony structure of that region. In several types of tumor, particularly the low-grade gliomas, MRI may show extensive brain infiltration in cases that fail to produce any image abnormality on CT or, at most, show a vague area of low density. Although either MRI or CT should be used with contrast enhancement in cases of suspected brain tumor, the passage of such contrast agents beyond the blood-brain barrier into the tissue does not necessarily imply the presence of a histologically malignant tumor. For example, although malignant gliomas almost always show contrast enhancement, so do meningiomas, which are entirely benign if they can be fully removed surgically.

CT scans done without contrast enhancement are of little value in the diagnosis of brain tumors or other mass lesions. Although it is true that hemorrhage, calcifications, hydrocephalus, and shift can be well seen on a non-contrast CT scan, the interpretation of even these conditions is tentative because each can have an underlying causative structural abnormality, such as a brain tumor, which may fail to appear on a non-contrast CT study. Allergy to CT dye is rare and is readily manageable. Currently available non-ionic CT dyes have an extremely low incidence of side effects. Currently used CT dyes carry little risk of causing renal dysfunction in normally hydrated patients who are not known to have kidney disease.

Brain tumors present in two patterns, not necessarily mutually exclusive. One consists of nonfocal symptoms of increased intracranial pressure, such as headaches, nausea, vomiting, confusion, and lethargy. The other consists of symptoms or signs of focal brain dysfunction, such as hemianopia, hemiparesis, cranial nerve palsies, or focal seizures. Such signs of focal brain dysfunction may have convincing localizing value even before an image of the brain is made by computed tomography (CT) or magnetic resonance imaging (MRI). Some tumors that arise in neurologically “silent” areas, such as the parietal or frontal association cortices, may produce only nonfocal generalized symptoms of headache, confusion, behavioral change, or, eventually, a seizure, despite growing to a considerable size. Although the capacity to reach early diagnosis by CT or MRI has greatly reduced the numbers of patients in whom symptoms of increased intracranial pressure represent initial complaints, examples still remain, especially in association with fast-growing tumors and in children. The latter are particularly likely to have tumors in the posterior fossa that tend to obstruct spinal fluid pathways earlier than do supratentorial tumors. The tempo with which a brain tumor grows also influences the presenting symptoms. Despite the fixed space within the skull (once infantile sutures have closed), the human brain possesses a remarkable capacity to make room for a slowly growing tumor. Because of this, and even allowing for the relative rapidity of growth of aggressive brain tumors, such as glioblastomas, the patient usually appears better clinically than might be expected from the degree of abnormality seen on CT or MRI scan.

FOCAL CLINICAL MANIFESTATIONS OF BRAIN TUMORS

These figures, given in parentheses, can be extremely variable from one center to another, depending on referral pattern. They are given here as general estimates based upon many published series.

WORLD HEALTH ORGANIZATION CLASSIFICATION OF BRAIN TUMORS:

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This is one of several formal schemes that are based on neuropathologic criteria. Metastasis is not considered, and one can get no sense of a given tumor as a clinical problem, as suggested by the simple classification in previous table.

GENERAL CONSIDERATIONS

“Is it benign or malignant?” is invariably the first question asked by patients, families, and physicians when confronted with a diagnosis of brain tumor. About a third of primary brain tumors can be called benign. Meningiomas and acoustic neuromas are good examples. They grow slowly, often can be removed completely, and rarely recur.
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The concept of malignancy in the central nervous system (CNS) has a different meaning from that which applies to systemic cancers. The term “malignant” has nothing to do with metastasis out of the CNS, which is extraordinarily rare. It has everything to do with anatomic location and the possibility of complete surgical removal. Unless a tumor can be completely excised to the last cell, all intracranial neoplasms are potentially malignant in that they may recur, and often do.