Intracranial neoplasms can arise from any of the structures or cell types present in the cranial vault, including the brain, meninges, pituitary gland, skull, and even residual embryonic tissue. The overall annual incidence of primary brain tumors in the United States is 14 cases per 100,000 population. The most common primary brain tumors are meningiomas, representing 27% of all primary brain tumors, and glioblastomas, representing 23% of all primary brain tumors; many of these tumors are clinically aggressive and high grade. Primary brain tumors are the most common of the solid tumors in children and the second most frequent cause of cancer death after leukemia in children. Brain metastases occur in approximately 15% of cancer patients as a result of hematogenous dissemination of systemic cancer, and the incidence may be rising due to better control of systemic disease. Lung and breast cancers are the most common solid tumors that metastasize to the central nervous system (CNS). Melanoma and testicular and renal carcinoma have the greatest propensity to metastasize to the brain, but their relative rarity explains the low incidence of these neoplasms in large series of patients with brain metastases. Patients with brain metastases from nonpulmonary primaries have a 70% incidence of lung metastases. Although many physicians presume that all brain metastases are multiple, in fact, half are single and many are potentially amenable to focal therapies. Epidemiology Gender There is a slight predominance of primary brain tumors in men. Age Primary brain tumors have a bimodal distribution, with a small peak in the pediatric population and a steady increase in incidence with age, beginning at age 20 years and reaching a maximum of 20 cases per 100,000 population between the ages of 75 and 84 years. Etiology and risk factors The cause of primary brain tumors is unknown, although genetic and environmental factors may contribute to their development. Genetic factors Clear heritable factors play a minor role in the genesis of primary brain tumors; less than 5% of patients with glioma have a family history of brain tumor. Several inherited diseases, such as tuberous sclerosis, neurofibromatosis type I, Turcot syndrome, and Li-Fraumeni cancer syndrome, predispose patients to the development of gliomas. However, these tumors tend to occur in children or young adults and do not account for the majority of gliomas, which appear in later life. Loss of heterozygosity (LOH) on chromosomes 9p and 10q and p16 deletions are frequently observed in high-grade gliomas, with low-grade gliomas having the fewest molecular abnormalities. In oligodendrogliomas, 1p and 19q LOH is associated with significantly improved survival. Molecular markers of brain tumors can predict survival and will become increasingly important in the diagnosis and treatment of glioma. Environmental factors Prior cranial irradiation is the only well-established risk factor for intracranial neoplasms. Lifestyle characteristics Brain tumors are not associated with lifestyle characteristics, such as cigarette smoking, alcohol intake, or cellular phone use. Signs and symptoms Brain tumors produce both nonspecific and specific signs and symptoms. Nonspecific symptoms include headaches, which occur in about half of patients, but are rarely an isolated finding of metastatic disease, and nausea and vomiting, which are caused by an increase in intracranial pressure. Because of the widespread availability of CT and MRI, papilledema is now seen in < 10% of patients, even when symptoms of raised intracranial pressure are present. Specific signs and symptoms are usually referable to the particular intracranial location of the tumor and are similar to the signs and symptoms of other intracranial space-occupying masses. Lateralizing signs, including hemiparesis, aphasia, and visual-field deficits, are present in ~50% of patients with primary and metastatic brain tumors. Seizures are a common presenting symptom, occurring in ~25% of patients with high-grade gliomas, at least 50% of patients with low-grade tumors, and 50% of patients with metastases from melanoma, perhaps due to their hemorrhagic nature. Otherwise, seizures are the presenting symptom in 15%-20% of patients with brain metastases. Seizures may be generalized, partial, or focal. Stroke-like presentation Hemorrhage into a tumor may present like a stroke, although the accompanying headache and alteration of consciousness usually suggest an intracranial hemorrhage rather than an infarct. Hemorrhage is usually associated with high-grade gliomas, occurring in 5%-8% of patients with glioblastoma multiforme. However, oligodendrogliomas have a propensity to bleed, and hemorrhage occurs in 7%-14% of these low-grade neoplasms. Sudden visual loss and fatigue may be seen with bleeding from pituitary tumors, termed pituitary apoplexy. Altered mental status Approximately 75% of patients with brain metastases have impairment of consciousness or cognitive function. Some patients with multiple bilateral brain metastases may present with an altered sensorium as the only manifestation of metastatic disease; this finding can be easily confused with metabolic encephalopathy. Screening for metastatic brain tumors Screening for brain metastases is performed in only a few clinical situations. Lung cancer Approximately 10% of patients with small-cell lung cancer (SCLC) have brain metastases at diagnosis, and an additional 20%-25% develop such metastases during their illness. Therefore, cranial CT or MRI is performed as part of the evaluation for extent of disease. Occasionally, patients with non-small-cell lung cancer (NSCLC) undergo routine cranial CT or MRI prior to definitive thoracotomy, since the presence of brain metastases may influence the choice of thoracic surgical procedure. This approach is particularly valuable in patients with suspected stage IIB or III disease for whom thoracotomy is considered following neoadjuvant therapy. Screening for brain metastases is both clinically worthwhile and cost-effective. Diagnosis MRI The diagnosis of a brain tumor is best made by cranial MRI. This should be the first test obtained in a patient with signs or symptoms suggestive of an intracranial mass. MRI is superior to CT and should be performed in a patient with signs or symptoms of an intracranial mass and whenever feasible in any patient being evaluated for metastatic brain disease (Figure 1). A high-quality, contrast-enhanced MR scan should be obtained to define the number of metastatic nodules and to look for evidence of leptomeningeal disease. For primary brain tumors, the MRI scan should always be obtained both with and without contrast material (gadolinium). High-grade or malignant primary brain tumors appear as contrast-enhancing mass lesions that arise in white matter and are surrounded by edema (Figure 2). Multifocal malignant gliomas are seen in ~5% of patients. Low-grade gliomas typically are nonenhancing lesions that diffusely infiltrate and tend to involve a large region of the brain. Low-grade gliomas are usually best appreciated on T2-weighted or fluid-attenuated inversion recovery (FLAIR) MRI scans (Figure 3). CT A contrast-enhanced CT scan may be used if MRI is unavailable or the patient cannot undergo MRI (eg, because of a pacemaker). CT is adequate to exclude brain metastases in most patients, but it can miss low-grade tumors or small lesions located in the posterior fossa. Tumor calcification is often better appreciated on CT than on MRI. PET Body positron emission tomography (PET) scans have a sensitivity of only 75% and a specificity of 83% for identification of cerebral metastases. Therefore, they are less accurate than MRI, which remains the gold standard. Radiographic appearance of lesions On CT or MRI, most brain metastases are enhancing lesions surrounded by edema, which extends into the white matter (Figure 1). Unlike primary brain tumors, metastatic lesions rarely involve the corpus callosum or cross the midline. The radiographic appearance of brain metastases is nonspecific and may mimic other processes, such as infection. Therefore, the CT or MRI scan must always be interpreted within the context of the clinical picture of the individual patient, particularly since cancer patients are vulnerable to opportunistic CNS infections or may develop second primaries, which can include primary brain tumors. Pathology Glial tumors arise from astrocytes, oligodendrocytes, or their precursors and exist along a spectrum of malignancy. The astrocytic tumors are graded, using a three-tier system, into astrocytoma, anaplastic astrocytoma, and glioblastoma multiforme. Grading is based on pathologic features, such as endothelial proliferation, cellular pleomorphism, mitoses, and necrosis. Low-grade astroglial tumors (such as astrocytoma and oligodendroglioma) and mixed neuronal-glial tumors (such as ganglioglioma) grow slowly but have a propensity to transform into malignant neoplasms over time. Transformation is usually associated with progressive neurologic symptoms and the appearance of enhancement on MRI. The high-grade gliomas include glioblastoma, gliosarcoma, anaplastic astrocytoma, and anaplastic oligodendroglioma. These tumors are extremely invasive, with tumor cells often found up to 4 cm away from the primary tumor mass. Ependymomas Intracranial ependymomas are relatively rare, accounting for < 2% of all brain tumors. They are most frequently seen in the posterior fossa or spinal cord, although they may also arise in the supratentorial compartment. Ependymomas are typically low grade histologically, but their high rate of recurrence indicates malignant behavior. Medulloblastomas are uncommon in adults but are one of the two most common primary brain tumors in children (the other being cerebellar astrocytomas). Medulloblastomas arise in the cerebellum and are always high-grade neoplasms.
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