Tumors of the Central Nervous System

Brain Imaging

Magnetic resonance imaging (MRI) should be performed both without and with intravenous administration of gadolinium, preferably using a standardized brain tumor imaging protocol. A well-performed MRI identifies any intracranial tumor, and a normal MRI effectively excludes a neoplasm.

The MRI of some extra-axial tumors (e.g., acoustic neuromas, meningiomas) is so characteristic that histologic confirmation is not required. A non–contrast-enhancing infiltrative lesion that is visible primarily on T2-weighted or fluid-attenuated inversion recovery images is most consistent with a low-grade glioma ( Fig. 175-1 ). A contrast-enhancing lesion with an area of central necrosis and surrounding edema is most likely to be a glioblastoma or possibly a brain metastasis.

MRI is far superior to computed tomography (CT) for the diagnosis of brain tumors and should be obtained in all patients with a suspected intracranial tumor. A CT scan, even with the administration of contrast material, may miss low-grade tumors and tumors in the posterior fossa, so a CT scan should be obtained only in patients who cannot undergo MRI.

In less clear cases, functional imaging techniques can be helpful to distinguish a brain tumor from other CNS lesions. Examples include cerebral blood volume (perfusion MRI), loss of tumor cellularity (diffusion-weighted MRI), the flow of contrast agent across a damaged blood-brain barrier (dynamic contrast-enhanced MRI), or the abundance of certain metabolites (MR spectroscopy). Malignant tumors, for example, are associated with an increased choline : N -acetylaspartate ratio and may contain areas with elevated levels of lactate and lipid. Functional MRI, which measures cerebral blood flow when areas of cortex are activated, may guide a safer and more complete surgical resection by localizing critical neurologic functions and their relationship to the tumor. Angiography no longer has a diagnostic role, but angiographic embolization is occasionally useful preoperatively to reduce the vascularity of some meningiomas, thereby making a complete resection safer.

Biopsy

Biopsy is typically part of a surgical procedure to remove the tumor to the greatest degree that is safe. However, most lesions that cannot be resected are still amenable to biopsy for diagnostic purposes. Stereotactic biopsy can reach lesions in almost any area of the brain with minimal morbidity. The risks of stereotactic biopsy include an inadequate tissue sample to make a diagnosis, a tissue sample that does not accurately reflect the most malignant grade of the tumor, and a procedure-related complication. Hemorrhage that causes neurologic impairment occurs in only 2% of stereotactic biopsies, typically in patients with glioblastomas. Unfortunately, some tumors, such as brain stem gliomas, are in such critical locations that biopsy should not be attempted.

Other Diagnostic Approaches

Electroencephalography (EEG) can be useful when a patient with a brain tumor has prolonged or unexplained confusion or stupor and for whom nonconvulsive status epilepticus is a consideration. Intraoperative monitoring is used frequently to help guide resection of epileptogenic cortex adjacent to or within the brain tumor tissue or to perform intraoperative mapping of critical functions to facilitate a safe, complete resection.

Cerebrospinal fluid analysis has a limited role in the diagnosis of most intracranial neoplasms. In primary CNS lymphoma ( Chapter 171 ), however, the diagnosis may be established on cerebrospinal fluid cytologic examination in about 15% of patients. The sensitivity of cerebrospinal fluid cytology in the diagnosis of CNS lymphoma increases when it is combined with flow cytometry and is further enhanced by immunophenotypic and molecular genetic analyses of the cerebrospinal fluid. Rarely, a lumbar puncture is required to exclude inflammatory conditions or other processes that may be confused with a primary brain tumor. Lumbar puncture must be avoided in patients with cerebellar tumors because the release of pressure through the spinal needle may result in herniation of the cerebellar tonsils through the foramen magnum. The ability to detect tumor-derived DNA and circulating tumor cells in cerebrospinal fluid from some patients with CNS tumors may be helpful to monitor the tumor’s evolution and response to therapy without the need for neurosurgical procedures.

Differential Diagnosis of Brain Tumors

Patients who present with symptoms of raised intracranial pressure or the new onset of central neurologic symptoms, such as hemiparesis or seizure, must be evaluated urgently. Prompt neuroimaging discloses a mass, and the radiographic features narrow the differential diagnosis ( Table 175-3 ). Extra-axial tumors, such as a meningioma or acoustic neuroma, can be confused with a dural metastasis. Low-grade intra-axial tumors, which are nonenhancing on MRI, have been confused with infections such as herpes encephalitis ( Chapter 383 ) when they involve the temporal lobe. Contrast-enhancing intra-axial tumors can be confused with a stroke ( Chapter 376 ), brain abscess ( Chapter 382 ), or focal plaque of demyelination ( Chapter 380 ). Subacute infarction can show brisk contrast enhancement, usually in a gyral pattern, unlike brain tumors in which enhancement is primarily in the white matter; however, the two are occasionally indistinguishable radiographically. Brain abscesses ( Fig. 382-1 ) typically have a thinner enhancing wall than a malignant tumor and have restricted diffusion. Despite careful evaluation, patients thought to have a malignant glioma occasionally are found at surgery to have a brain abscess. A single large plaque of demyelination can also be confused radiographically with a brain tumor and sometimes the diagnosis can be established only by biopsy.

When MRI suggests a primary brain tumor, an extensive systemic search for a possible source of metastasis is rarely required. Brain metastases are more common than primary brain tumors, but most occur in patients with known cancer, typically with active systemic disease. If an obvious systemic cancer is not revealed by a thorough physical examination, chest radiograph, routine blood tests, and urinalysis, the patient should proceed to craniotomy. Even if a brain metastasis is found at surgery, resection of a single brain metastasis is the appropriate treatment, and the pathologic examination of the lesion guides the subsequent search for the primary tumor.