Metastatic brain cancer, which is also called secondary brain cancer, occurs when cancer cells spread to the brain from a primary cancer elsewhere in the body, and form a tumor or tumors there. This type of brain cancer is about ten times more common than cancer that starts in the brain, which is known as primary brain cancer.
Each year about 100,000 people in the United States are diagnosed with brain metastases. Five to 25 percent of cancer patients will develop metastases in the brain -- the wide range explained by the fact that metastasis depends entirely upon the patient's type of primary tumor. For example, one of the most common primary tumors that spread to the brain is malignant melanoma, metastasizing in nearly 50 percent of cases. On the other hand, gastrointestinal cancers (those cancers originating in the digestive system) spread to the brain less than ten percent of the time. The outlook for patients with brain metastases generally depends on the number, size, location, and origin of the primary tumor(s).
Common Origins of Brain Metastases
Although brain metastases can develop from almost any kind of cancer, those originating in the lung, breast, colon, and kidney, along with malignant melanoma are the most likely to metastasize to the brain. About half of patients with brain metastases have more than one tumor in the brain.
Physicians believe that the number of patients diagnosed with brain metastases is on the rise because treatments for primary cancers are improving continually and patients are surviving with these diseases longer. Brain lesions can also be more easily detected now because of recent advances in diagnostic techniques.
How Cancer Cells Travel to the Brain
Cancer cells can break away from the primary tumor site and travel through blood and lymphatic vessels. This is how cancer cells spread, or metastasize, to another part of the body, such as the brain.
Metastases most often appear in the brain at the junction of two types of brain tissue called gray matter and white matter. This junction is rich with blood vessels of very narrow caliber, and metastatic cells often lodge there. Gray matter makes up the outer layer of the brain and contains cells called neurons. White matter is composed of axons, which connect neurons to one another and are sheathed in a white fat called myelin. Gray matter is where computational thinking occurs, and white matter is responsible for communication between groups of cells in different areas of the brain.
In most patients with brain metastases, tumors appear in the cerebral cortex, the two large hemispheres of the brain where most high-level functions (such as consciousness, memory, language, and sensory perception) are governed. Fifteen percent of brain metastases develop in the cerebellum, where complex voluntary muscle movements are regulated and coordinated. Five percent of metastatic tumors develop in the brainstem, where functions such as visual coordination, swallowing, and balance are directed.
In a small number of patients, brain metastases appear before the primary cancer is discovered in another part of the body. This is called a metastasis of unknown origin. These tumors can develop when a patient's primary cancer, while still undetectable at its original site, sends out metastatic cells that travel to the brain and establish themselves there. In these patients, physicians can sometimes biopsy the tumor (depending on its location in the brain), identify the type of cells it is composed of, and determine its site of origin.
