What causes brain tumors? Here's why they're not that common.

Tumors are one of the most misunderstood diagnoses or conditions. While no one wants to be told they have one, many people don't realize that a tumor is nothing more than a clump of abnormal cells that form a mass. Such masses can certainly become very large, but they can also be so small that even an unusual pimple or a mole qualifies - which is why doctors sometimes recommend having these looked at or removed by a dermatologist. 

Regardless of a tumor's size, the most concerning aspect of having one is whether it is cancerous, also known as malignant. Fortunately, about 90% of tumors are noncancerous, also known as benign. 

Of the many different types of malignant or benign tumors out there, malignant brain tumors are considered among the rarest. "Less than 1% of the population will be diagnosed with a malignant brain tumor in their lifetime," says Dr. Marissa Barbaro, a neuro-oncologist at the Perlmutter Cancer Center at NYU Langone Hospital—Long Island.

Still, understanding what brain tumors are and what causes them can be helpful in taking steps to prevent them. 

What is a brain tumor? 

As with any type of tumor, brain tumors are an abnormal growth of cells, the number of which will determine, in part, the overall size of the tumor. In the case of benign brain tumors, these cells usually grow slowly and won't spread to other areas of the body - causing the tumor to have distinct borders.

Malignant brain tumors, on the other hand, can spread very quickly, often invading surrounding brain structures. "These tumors tend to be more aggressive and can be very difficult to treat and eradicate," says Dr. Andrea Cercek, a gastrointestinal oncologist and co-director of the Center for Young Onset Colorectal and Gastrointestinal Cancers at Memorial Sloan Kettering Cancer. 

Barbaro explains that there are two main types of brain tumors: primary brain tumors, "which are tumors that arise from the brain itself;" and secondary brain tumors, "which are tumors that spread to the brain from a cancer in another part of the body such as lung or breast cancer." Secondary tumors are much more common than primary brain tumors, "as only approximately 90,000 people are diagnosed with a primary brain tumor each year," she explains.

Regardless of whether a brain tumor is primary or secondary, it can become problematic. While malignant brain tumors are more concerning due to the fact that they are difficult to contain and can spread very quickly, benign brain tumors are often more worrisome than benign tumors discovered on or in other parts of the body.

This is because even if they grow slowly and don't spread the way malignant tumors do, benign brain tumors can still compress different areas of the brain. "The brain controls every process that regulates our body including thought, memory, emotion, touch, motor skills, vision, breathing, temperature and hunger," says Dr. Julie Gralow, the chief medical officer at the American Society of Clinical Oncology. "Brain tumors can impact all of these functions if a mass starts taking up space inside the skull and increases pressure on the brain," she explains. "This can potentially lead to neurologic conditions such as muscle weakness or paralysis, speech problems, and seizures."

Because of this, doctors usually recommend surgically removing both malignant and benign brain tumors, once discovered. 

What causes brain tumors? 

Many of the factors that contribute to a tumor forming in any other part of the body can contribute to the formation of a brain tumor. One significant factor is related to age as it's been shown that the immune system declines as we get older and waning immune health means tumor cells are less likely to be identified and killed. This is why brain tumors are far more common in elderly adults than younger ones and why people with immune system disorders are also most suspectable to developing brain tumors.

While more research is still needed, environmental factors including radiofrequency, electromagnetic waves and air pollution and related exposure to environmental toxins may play a part as well. But the most clearly understood environmental risk factor "is exposure to ionizing radiation, especially at a young age," says Barbaro. "For example, children who had radiation to the head as part of treatment for a childhood cancer like leukemia, or people who have been exposed to a nuclear disaster." 

Gralow says one's family history and genetics may also increase the risk of developing a brain tumor. Whether a person has experienced a brain injury before might also contribute. Research shows that diet may play a role as well. 

Can brain tumors be prevented?  

While no tumor can be prevented completely, certain steps can reduce one's risk of developing a brain tumor. These include avoiding smoking and excessive radiation exposure, plus steering clear of any environmental hazards. Children and pregnant women should also avoid radiation exposure such as CT scans to the head unless absolutely needed, per the American Cancer Society. 

Keeping one's immune system strong is also important. Immune health can be improved by getting enough sleep, minimizing stress, and eating a well-balanced diet. 

How are brain tumors usually found?

When brain tumors aren't prevented and end up forming, some symptoms that can lead to their discovery include speech problems, weakness or numbness on one side of the body, frequent headaches, changes in vision, and loss of balance or feeling unsteady. If such symptoms persist or increase, Gralow says a visit to the doctor is in order.

Once a growth of cells has been discovered in or around the brain, malignancy is determined through a pathology report or biopsy, says Barbaro. "When this occurs," she explains, "advanced molecular testing and other detailed pathologic analysis is done on the tissue, which determines the specific tumor type and how aggressively it is expected to behave."