Whether primary, metastatic, malignant, or benign, brain tumors must be differentiated from other space-occupying lesions such as abscesses, arteriovenous malformations, and infarction, which can have a similar clinical presentation. Other clinical presentations of brain tumors include focal cerebral syndromes such as seizures. Seizures are a presenting symptom in approximately 20% of patients with supratentorial brain tumors and may antedate the clinical diagnosis by months to years in patients with slow-growing tumors. Among all patients with brain tumors, 70% with primary parenchymal tumors and 40% with metastatic brain tumors develop seizures at some time during the clinical course.
Computed tomography (CT) and magnetic resonance imaging (MRI) have complementary roles in the diagnosis of CNS neoplasms.[8,10] The speed of CT is desirable for evaluating clinically unstable patients. CT is superior for detecting calcification, skull lesions, and hyperacute hemorrhage (bleeding less than 24-hours old) and helps direct differential diagnosis as well as immediate management. MRI has superior soft-tissue resolution. MRI can better detect isodense lesions, tumor enhancement, and associated findings such as edema, all phases of hemorrhagic states (except hyperacute), and infarction. High-quality MRI is the diagnostic study of choice in the evaluation of intramedullary and extramedullary spinal cord lesions. In post-therapy imaging, single-photon emission computed tomography (SPECT) and positron emission tomography (PET) may be useful in differentiating tumor recurrence from radiation necrosis.
Biopsy confirmation to corroborate the suspected diagnosis of a primary brain tumor is critical, whether before surgery by needle biopsy or at the time of surgical resection, except in cases where the clinical and radiologic picture clearly point to a benign tumor. Radiologic patterns may be misleading, and a definitive biopsy is needed to rule out other causes of space-occupying lesions, such as metastatic cancer or infection. CT- or MRI-guided stereotactic techniques can be used to place a needle safely and accurately into all but a very few inaccessible locations within the brain.
Specific genetic or chromosomal abnormalities involving deletions of 1p and 19q have been identified for a subset of oligodendroglial tumors, which have a high response rate to chemotherapy.[2,7,11,12,13,14,15] Other CNS tumors are associated with characteristic patterns of altered oncogenes, altered tumor-suppressor genes, and chromosomal abnormalities. Familial tumor syndromes with defined chromosomal abnormalities are associated with gliomas. (Refer to the Classification section of this summary for more information.)
Metastatic Brain Tumors
Brain metastases outnumber primary neoplasms by at least 10 to 1, and they occur in 20% to 40% of cancer patients. Because no national cancer registry documents brain metastases, the exact incidence is unknown, but it has been estimated that 98,000 to 170,000 new cases are diagnosed in the United States each year.[2,8] This number may be increasing because of the capacity of MRI to detect small metastases and because of prolonged survival resulting from improved systemic therapy.[2,16]