Patients who present with symptoms and signs of increased intracranial pressure or a first convulsive seizure need to be hospitalized. Diagnosis and treatment measures must be started at once; it may be unsafe to wait. Those who present with focal neurologic impairment and who do not have symptoms of increased intracranial pressure may reasonably be evaluated in the outpatient setting for other conditions that are often considerations in the differential diagnosis of brain tumor. The tempo of evolution of symptoms and signs of focal neurologic impairment, much more than their severity, governs urgency of evaluation. The tempo also strongly influences diagnostic considerations. Although an occasional brain tumor may manifest with such rapid onset of hemiparesis or aphasia that a stroke is mimicked, most do not. Associated aspects of the history, such as recent head trauma, previous episodes of reversible neurologic impairment, or recent infection and fever, should direct attention to diagnostic alternatives such as subdural hematoma, multiple sclerosis, or cerebral abscess. Simply stated, it is the careful history, not the neurologic examination, that usually points to the alternative diagnoses.

IMAGING AND OTHER DIAGNOSTIC PROCEDURES

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.

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.

THE MAIN DIFFERENTIAL DIAGNOSES OF BRAIN TUMORS: