Computed tomography (CT) and magnetic resonance imaging (MRI) have complementary roles in the diagnosis of CNS neoplasms.[11,13] The speed of CT is desirable for evaluating clinically unstable patients; it 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; it 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 posttherapy imaging, single-photon emission computed tomography (SPECT) and positron emission tomography (PET) may be useful in differentiating tumor recurrence from radiation necrosis.
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 lomustine, procarbazine, and vincristine (PCV) therapy.[7,14,15,16,17,18] Other CNS tumors are associated with characteristic patterns of altered oncogenes, altered tumor-suppressor genes, and chromosomal abnormalities. As noted above, 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.[2,11] 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.[2,19]
The most common primary cancers metastasizing to the brain are lung cancer (50%), breast cancer (15%-20%), unknown primary cancer (10%-15%), melanoma (10%), and colon cancer (5%).[19,20] Eighty percent of brain metastases occur in the cerebral hemispheres, 15% occur in the cerebellum, and 5% occur in the brain stem. Metastases to the brain are multiple in more than 70% of cases, but solitary metastases also occur. Brain involvement can occur with cancers of the nasopharyngeal region by direct extension along the cranial nerves or through the foramina at the base of the skull. Dural metastases may constitute as much as 9% of total CNS metastases.
A lesion in the brain should not be assumed to be a metastasis just because a patient has had a previous cancer; such an assumption could result in overlooking appropriate treatment of a curable tumor. Primary brain tumors rarely spread to other areas of the body, but they can spread to other parts of the brain and to the spinal axis.