Introduction:

Brain tumors are composed of cells that exhibit unrestrained growth in the brain.

The major areas of the brain have one or more specific functions.

They can be benign (noncancerous, meaning that they do not spread elsewhere or invade surrounding tissue) or malignant (cancerous).

Cancerous brain tumors are further classified as either primary or secondary tumors.

Primary Brain Tumors

Primary tumors start in the brain, whereas secondary tumors spread to the brain from another site such as the breast or lung. (In this report, the term "brain tumor" will refer mainly to primary malignant tumors, unless otherwise specified.)

Benign Brain Tumors. Benign tumors represent half of all primary brain tumors. Their cells look relatively normal, grow slowly, and do not spread (metastasize) to other sites in the body or invade brain tissue. Benign tumors can still be serious and even life-threatening if they are in vital areas in the brain where they exert pressure on sensitive nerve tissue or if they increase pressure within the brain. While some benign brain tumors may pose a health risk, including risk of disability and death, most are usually successfully treated with techniques such as surgery.

Malignant Brain Tumors. A primary malignant brain tumor is one that originates in the brain itself. Although primary brain tumors often shed cancerous cells to other sites in the central nervous system (the brain or spine), they rarely spread to other parts of the body.

Brain tumors are generally named and classified according to the following:

• The type of brain cells from which they originate, or
• The location in which the cancer develops

The biologic diversity of these tumors, however, makes classification difficult.

Secondary (Metastatic) Malignant Brain Tumors

A secondary (metastatic) brain tumor occurs when cancer cells spread to the brain from a primary cancer in another part of the body. Secondary tumors are about three times more common than primary tumors of the brain. Usually, multiple tumors develop. Solitary metastasized brain cancers may occur but are less common. Most often, cancers that spread to the brain to cause secondary brain tumors originate in the lung, breast, kidney, or from melanomas in the skin.

All metastatic brain tumors are malignant. This report discusses primary malignant brain tumors.

Primary Glioma Brain Tumors

About 80% of malignant primary brain tumors are known collectively as gliomas. Gliomas are not a specific type of cancer but are a term used to describe tumors that originate in glial cells. Glial cells are the building-block cells of the connective, or supportive, tissue in the central nervous system.

Gliomas are classified into four grades that reflect the degree of malignancy. Grades I and II are considered low-grade and grades III and IV are considered high-grade. Grade I and II are the slowest-growing and least malignant; grade I tumors are generally considered borderline between benign and malignant. Grade III tumors are considered malignant and grow at a moderate rate. Grade IV tumors, such as glioblastoma multiforme, are the fastest-growing and most malignant primary brain tumors.

There are several glial cell types from which gliomas form. The names of these gliomas and their cell types are:

Astrocytomas are primary brain tumors derived from astrocytes, which are star-shaped glial cells. Astrocytomas account for about 60% of all malignant primary brain tumors. Astrocytoma tumor types by grade include:

• Grade I. Pilocytic astrocytoma is one of the most common types of glioma in children
• Grade II. Diffuse astrocytoma (also called low-grade astrocytoma) typically occurs in men and women ages 20 - 60
• Grade III. Anaplastic astrocytoma typically occurs in adults ages 30 - 60 and is more common among men than women.
• Grade IV. Glioblastoma multiforme (GBM) accounts for about 50% of all astrocytomas. These highly malignant aggressive tumors are most common in older adults (50s - 70s), particularly men. Only about 10% of childhood brain tumors are glioblastomas.

Oligodendrogliomas develop from oligodendrocyte glial cells, which form the protective coatings around nerve cells. Oligodendrogliomas are classified as either low-grade (grade II) or anaplastic (grade III). Pure oligodendrogliomas, however, are rare. In most cases they occur in mixed gliomas. Oligodendrogliomas usually occur in younger and middle-aged adults.

Ependymomas are derived from ependymal cells, which line the ventricles (fluid-filled cavities) in the lower part of the brain and the central canal of the spinal cord. They are one of the most common type of brain tumor in children. They can also occur in adults in their 40s and 50s. Ependymomas are divided into four categories: Myxopapillar ependymomas (grade I), subependymomas (grade I), ependymomas (grade II), and anaplastic aependymomas (grades III and IV).

Mixed gliomas contain a mixture of malignant gliomas. About half of these tumors contain cancerous oligodendrocytes and astrocytes.

Gliomas may also contain cancer cells derived from brain cells other than glial cells.

Location of Gliomas. Gliomas are also described by the location of the tumor. Examples include:

• Brain stem gliomas develop in the lowest portion of the brain. The brain stem connects the cerebrum (the higher centers of the brain) to the spinal cord. Between 10 - 20% of brain tumors in children are brain stem gliomas. Most of these tumors are astrocytomas.
• Cerebellar astrocytomas occur in the cerebellum part of the brain, which controls balance and coordination.
• Optic gliomas occur in the optic nerve and other parts of the eye. They primarily affect children younger than age 10. About 20% of children with the genetic disorder neurofibromatosis 1 (NF-1) develop optic gliomas. Pilocytic astrocytoma and fibrillary astrocytoma are common types of optic gliomas.

Primary Non-Glioma Brain Tumors

Malignanttypes of non-glioma brain tumors include:

Medulloblastomas. Medulloblastomas are always located in the cerebellum, which is at the base and toward the back of the brain. These fast-growing high-rade tumors represent about 15 - 20% of pediatric brain tumors and 20% of adult brain tumors.

Pituitary Adenomas. Pituitary tumors (called pituitary adenomas) comprise about 10% of primary brain tumors and are often benign, slow-growing masses in the pituitary gland. They are more common in women than men.

Central Nervous System Lymphomas. Central nervous system (CNS) lymphomas can affect both people with healthy immune systems and those who are immunocompromised due to other medical conditions (recipients of organ transplants, patients infected with HIV). CNS lymphomas occur most often in the cerebral hemisphere but can also develop in cerebrospinal fluid, eyes, and spinal cord. [For more information, see In-Depth Report #84: Non-Hodgkin’s lymphoma.]

Benign types of non-glioma brain tumors include:

Meningiomas. Meningiomas are usually benign tumors that develop in the membranes that cover the brain and spinal cord (the meninges).

Meningiomas account for about 25% of all primary brain tumors and are most common in women in their 60s and 70s. Meningiomas are classified as benign meningioma (grade I), atypical meningioma (grade II), and anaplastic meningioma (grade III).