A brain tumor starts when cells in the brain change and grow out of control. As they continue to grow, they form a mass of cells that becomes a tumor. Brain tumors form in 1 of 2 ways:

Primary brain tumor. This starts with an abnormal (mutated) brain cell that grows in the brain.

Metastatic (secondary) tumor. This starts as a cancer in another part of the body, such as the lungs or breast. It then spreads to the brain, where it forms a new tumor. The cancer cells in the brain still look like the cells where they first started, like the lung or breast. They don’t look like mutated brain cells.

Secondary brain tumors are much more common than primary brain tumors in adults. Primary brain tumors are more common in children.

Healthcare providers don’t know why some cells mutate and form tumors. It may have something to do with a person’s genes, or their environment, or both.

Brain tumors can cause damage by growing into and pushing on key areas of the brain. They can also cause problems if they block the flow of fluid around the brain. This fluid is called cerebrospinal fluid (CSF). This can lead to an increase in the pressure inside the skull. Some types of tumors can spread through the CSF to other parts of the brain or the spinal cord.

A primary brain tumor can be cancer (malignant) or not cancer (benign):

A malignant primary tumor is more dangerous because it can grow quickly. It may grow into or spread to other parts of the brain or to the spinal cord. Malignant tumors are also sometimes called brain cancer. (Metastatic brain tumors are always cancer. They have spread to the brain from cancer in another part of the body.)

A benign primary brain tumor is not cancer. Benign tumors can cause damage by growing into and pressing on other parts of the brain. But these tumors don’t spread. (In some cases, a benign tumor can become more aggressive over time.)

Brain tumor symptoms

Symptoms of a brain tumor depend on how big the tumor is and where it is in the brain. Symptoms may also be affected by increased pressure in the brain from the mass or from swelling. They may include:

Headache

Dizziness

Weakness or numbness in the arms, legs, or face

Nausea or vomiting

Changes in speech, vision, hearing, memory, or personality

Seizures

Feeling tired all the time (fatigue)

Problems with balance or walking

Types of brain tumors

The type of brain tumor, the size of the tumor, and its location within the brain are important. For instance, some benign (not cancer) brain tumors can be quite harmful. They can cause severe problems if in or near a key part of the brain. There are more than 100 types of brain tumors. They’re often named by the type of brain cell or part of the brain where they start.

Click to Enlarge

Some of the more common types of primary brain tumors in adults include:

Astrocytoma. This is the most common type of malignant (cancer) brain tumor. Its name comes from the star-shaped brain cells that make up the tumor. These tumors can grow anywhere in the brain.

Meningioma. This is the most common type of brain tumor in adults. They are usually not cancer (benign). These tumors start in the lining that covers the brain. They can cause serious problems when they press on vital parts of the brain or grow into nearby brain tissue. In rare cases, they can become cancer. They usually grow slowly and are only found by accident when an MRI is done for some other reason.

Oligodendroglioma. These brain tumors form in the cells that make the fatty lining that covers nerves (called myelin).

Astrocytomas are common in children, as they are in adults. But they are less likely to be cancer. These are other common primary brain tumors in children:

Medulloblastoma. This tumor is cancer. It forms in the cerebellum, the back part of the brain near the spinal cord. It can spread to the spinal cord and cause spinal fluid to build up into the brain (hydrocephalus).

Ependymoma. This rare tumor is found in young children and young adults. It may or may not be cancer. It starts in the lining around fluid-filled areas of the brain (called the ventricles).

Brain stem glioma. This tumor starts in the base of the brain. It often spreads through the normal tissue.

Metastatic or secondary brain tumors are more common than primary brain tumors in adults. These tumors are usually found in more than 1 part of the brain. The most common cancers that spread to the brain are lung, breast, colon, bladder, kidney, melanoma skin cancers, and leukemia and lymphoma.

Grading primary brain tumors

Most cancer centers that treat brain tumors use a grading system developed by the World Health Organization. It uses a scale of 1 to 4 and is written with Roman numerals I, II, III, and IV.

A tumor’s grade is determined by looking at cells from the tumor under a microscope. Tumor grading is important because it’s 1 factor that helps healthcare providers decide how to treat a tumor.

Grade I and II tumors are typically considered low grade. They look more like normal cells under the microscope. They’re less likely to spread and tend to be easier to treat. Grade III and IV tumors are considered high grade. They grow faster and are harder to treat. Over time, without treatment some low-grade tumors can become high-grade tumors.

Here is more on tumor grades:

Grade I. These tumors are slow growing and don’t spread into nearby tissue. They may be considered benign (not cancer). They can often be cured with surgery, and they rarely come back.

Grade II. These tumors grow slowly but can spread into nearby brain tissue. They’re more likely to come back after treatment than grade I tumors. They’re also more likely to start growing faster over time.

Grade III. These tumors tend to spread into nearby parts of the brain. They may need other treatments along with surgery.

Grade IV. These tumors grow and spread most rapidly. They are least likely to be cured.

If you or a loved one has been diagnosed with a brain tumor, keep in mind that many new treatments are available. These are leading to longer survival and better quality of life. Treatment choices depend on a person’s age, overall health, and the tumor type, grade, location, and other factors. Learn as much as you can about the type of brain tumor you have and work closely with your medical team to find the best treatment.