Most brain tumors have no clearly identifiable cause. They result from DNA changes inside brain cells that trigger uncontrolled growth, but pinpointing exactly why those changes happen in a given person is rarely possible. The confirmed risk factors, including radiation exposure, inherited genetic conditions, and cancer spreading from elsewhere in the body, account for only a fraction of cases. The median age at diagnosis is 61, and roughly 6 in every 100,000 people are diagnosed each year.
How a Brain Tumor Forms
Every cell in your body contains tightly packaged DNA organized into loops. Sections called insulators act like barriers, keeping genes in one loop from accidentally interacting with genes in a neighboring loop. When something disrupts this structure, genes that normally sit far apart can end up right next to each other. If a growth-promoting gene gets paired with a regulatory gene that keeps switching it on, the cell starts dividing without stopping.
One well-studied example involves a mutation in a gene called IDH, which is common in a type of brain tumor called a glioma. This mutation causes extra chemical tags (methyl groups) to attach to DNA throughout the genome. That process damages the insulator barriers, allowing the DNA loops to misfold. In one case, misfolding brings a regulatory gene into direct contact with a growth-promoting gene called PDGFRA, essentially jamming the cell’s growth signal into the “on” position. The key insight is that the cancer-causing gene itself isn’t broken. It’s structurally intact but being activated at the wrong time and place because of packaging defects in the DNA around it.
Ionizing Radiation Exposure
High-energy radiation is the most firmly established environmental cause of brain tumors. This includes radiation therapy to the head (often given during treatment for a previous cancer) and, to a lesser extent, repeated imaging scans. Data from the National Cancer Institute shows that cumulative radiation from two to three head CT scans in children, based on current scanner settings, could triple the risk of developing a brain tumor. That sounds alarming, but the baseline risk is low, so the actual number of additional cases is small.
The important distinction is between ionizing radiation (X-rays, gamma rays, radiation therapy) and non-ionizing radiation (cell phones, power lines, Wi-Fi). Ionizing radiation carries enough energy to directly damage DNA. Non-ionizing radiation does not. No mechanism by which cell phone signals could cause cancer has been identified, and a 2015 review by the European Commission’s scientific committee found that studies on radiofrequency exposure do not show an increased risk of brain tumors or other cancers of the head and neck.
Inherited Genetic Conditions
A small proportion of brain tumors are linked to inherited syndromes that make a person more vulnerable to abnormal cell growth throughout life. These conditions are rare, but if you have a family history of any of them, your risk of a brain tumor is higher than average.
- Neurofibromatosis type 1 and type 2: causes tumors to grow along nerves, including nerves in and around the brain
- Li-Fraumeni syndrome: a mutation in a key tumor-suppressing gene that raises the risk of many cancers, including brain tumors
- Tuberous sclerosis: causes noncancerous growths in the brain and other organs
- Von Hippel-Lindau syndrome: leads to tumors and cysts in multiple organs, including the central nervous system
- Turcot syndrome, Turner syndrome, and Gorlin syndrome: each associated with elevated brain tumor risk through different genetic pathways
Having one of these syndromes doesn’t guarantee a brain tumor will develop. It means the cellular safeguards that normally catch and repair DNA errors are weakened, giving abnormal cells a greater chance of surviving and multiplying.
Cancer That Spreads to the Brain
Not all brain tumors start in the brain. Secondary brain tumors, called brain metastases, form when cancer cells from another part of the body break away and travel through the bloodstream or lymphatic system to the brain. The most common cancers that spread to the brain are lung, breast, colon, kidney, and melanoma.
Brain metastases are actually more common than primary brain tumors in adults. The symptoms can be similar, including headaches, seizures, and cognitive changes, but the treatment approach differs because the underlying disease originated elsewhere.
Age and Sex
Brain tumors can occur at any age, but the risk rises significantly after 55. Nearly 23% of new cases are diagnosed in people aged 65 to 74, making this the highest-risk age group. About 10% of cases occur in people under 20, which is notable because brain tumors are one of the more common cancers in children, even though the absolute numbers are low.
Men are diagnosed at a higher rate than women: 7.1 per 100,000 for males compared to 5.1 per 100,000 for females. The reasons for this gap aren’t fully understood, though hormonal differences, occupational exposures, and genetic factors may all play a role.
Occupational and Chemical Exposures
Certain workplace exposures have been studied for a possible connection to brain tumors, though the evidence is more mixed than for radiation. One large study published through the CDC found that women who reported using herbicides had roughly 2.4 times the risk of developing meningioma (a tumor that forms in the membranes surrounding the brain) compared to women who never used herbicides. The risk increased with more years of exposure. The same study found that general farmers and farmworkers had about 2.5 times the risk of glioma.
These findings don’t apply uniformly. The herbicide-meningioma link was significant in women but not men, and insecticide exposure showed no clear association with brain tumors in either sex. Industrial solvents, vinyl chloride, and certain rubber manufacturing chemicals have also been investigated, though none with the same level of certainty as ionizing radiation.
Why Most Cases Have No Clear Cause
If you or someone you know has been diagnosed with a brain tumor, the natural instinct is to search for a reason. In the vast majority of cases, no single identifiable cause can be found. The DNA mutations that drive brain tumors often arise spontaneously during normal cell division, accumulating over a lifetime. This is partly why risk increases with age.
The known risk factors, radiation, inherited syndromes, metastatic cancer, and certain occupational exposures, account for a minority of diagnoses. For everyone else, the tumor likely results from a combination of random genetic errors and subtle environmental influences that current science can’t yet pinpoint at the individual level. That uncertainty is frustrating, but it also means that in most cases, there was nothing a person did or failed to do that caused the tumor to develop.