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, forming a tumor or tumors in the brain. 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 is 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 to spread to the brain is malignant melanoma. In nearly 50 percent of people with melanoma that has metastasized, the disease can be found in the brain. On the other hand, gastrointestinal cancers (those cancers originating in the digestive system) spread to the brain less than 10 percent of the time. The outlook for patients with brain metastases generally depends on the number, size, location, and origin of the primary tumor or tumors.
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 diameter, 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 brain stem, 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.
Symptoms of brain metastases are quite varied and depend on the location and size of the tumor or tumors. These symptoms can include:
About half the patients with brain metastases develop headaches. Headaches develop when the tumor or tumors create pressure inside the skull and compress surrounding brain tissue. This type of headache is typically at its worst in the morning and tends to improve over the course of the day. It may be accompanied by nausea and vomiting.
These occur when a tumor triggers an abnormal flow of electrical impulses through the brain. Patients with partial, or focal, seizures may experience twitching or jerking muscles, abnormal smells or tastes, problems with speech, or numbness and tingling. Generalized seizures can cause the patient to lose consciousness.
- Speech Problems, Comprehension Problems, Impaired Vision, Weakness or Numbness in Parts of the Body
These symptoms arise when a tumor affects the areas of the brain that govern speech, cognition, or muscle control.
- Motor Problems
These evolve when a tumor disrupts the normal flow of signals from the brain to the muscles.
Some patients with brain metastases have not experienced any symptoms at diagnosis. Instead, the tumor is found incidentally during tests for other conditions, or as part of staging done for a primary cancer.