Most brain cancers have no single identifiable cause. The lifetime risk of being diagnosed with a brain or nervous system tumor is about 0.6%, and for the majority of cases, the tumor arises from random genetic mutations in brain cells rather than from a clear trigger. That said, researchers have identified several factors that meaningfully raise or lower the odds, from radiation exposure to inherited syndromes to cancers that spread from elsewhere in the body.
Primary vs. Metastatic Brain Tumors
An important distinction comes first: not all brain cancer starts in the brain. Secondary (metastatic) brain tumors, which originate in another organ and spread to the brain, are actually more common than primary brain tumors in adults. The incidence of brain metastases is roughly double that of tumors that originate in the brain itself.
The cancers most likely to spread to the brain are lung cancer (responsible for 39 to 56% of brain metastases), breast cancer (13 to 30%), melanoma (8 to 11%), gastrointestinal cancers (6 to 9%), and kidney cancer (2 to 6%). If you’ve been told you have a brain tumor, one of the first things doctors determine is whether it started in the brain or traveled there, because the cause, treatment, and outlook differ significantly.
The rest of this article focuses on primary brain tumors, the ones that develop directly in brain tissue.
Ionizing Radiation Is the Strongest Known Risk Factor
Moderate-to-high-dose ionizing radiation is the only environmental risk factor that researchers consider firmly established for brain tumors. The strongest evidence comes from studies of atomic bomb survivors and people who received radiation to the head during childhood, either for other cancers or for conditions that were once treated with radiation.
The risk increase depends on the dose and the type of tumor. Meningiomas (tumors of the tissue surrounding the brain) appear especially sensitive to radiation, with risk estimates roughly 5 to 10 times higher per unit of dose than gliomas (tumors arising from brain cells themselves). Nerve sheath tumors called schwannomas also show a strong link, based on long-term studies of atomic bomb survivors.
Lower-dose exposures, like repeated dental X-rays or CT scans of the head, have shown weaker and less consistent associations. One large study found a twofold increase in meningioma risk among people who had six or more full-mouth dental X-ray series over a lifetime. Another found elevated glioma risk after three or more head CT scans, but only in people with a family history of cancer. These findings don’t mean a single scan is dangerous. They suggest that cumulative exposure over many years may matter for some people.
Genetic Mutations That Drive Tumor Growth
Brain tumors form when the DNA inside a brain cell changes in ways that let the cell grow uncontrollably. In most cases, these mutations happen spontaneously over a person’s lifetime and aren’t inherited. One well-studied example involves a gene called IDH, which normally helps cells process energy. When IDH mutates, it produces a molecule that essentially jams the cell’s ability to regulate its own genes. Chemical tags that are supposed to turn genes on and off get stuck in the wrong position, locking cells in an immature, rapidly dividing state. This pattern is characteristic of lower-grade gliomas and is one of the first things pathologists look for when classifying a brain tumor.
Other common mutations affect genes that control cell division (like TP53), suppress tumor growth (like PTEN), or regulate how chromosomes are maintained. No single mutation causes brain cancer on its own. It typically takes several mutations accumulating in the same cell before a tumor forms, which is one reason brain cancer becomes more common with age.
Inherited Syndromes
Only about 5% of gliomas are familial, meaning two or more members of the same family develop the disease. But several rare inherited syndromes carry a substantially elevated risk for specific types of brain tumors.
- Neurofibromatosis type 1 (NF1) primarily causes gliomas along the optic nerve pathway and brainstem. These are usually slow-growing pilocytic astrocytomas, most often diagnosed in children.
- Neurofibromatosis type 2 (NF2) is associated with schwannomas on the hearing and balance nerves, often on both sides, along with meningiomas.
- Li-Fraumeni syndrome is caused by inherited mutations in the TP53 tumor suppressor gene and raises the risk of aggressive gliomas (including glioblastoma), as well as breast cancer, sarcomas, and leukemia.
- Von Hippel-Lindau disease leads to hemangioblastomas, blood vessel tumors that typically form in the back of the brain or upper spinal cord.
- Tuberous sclerosis is linked to a specific type of slow-growing brain tumor called a subependymal giant cell astrocytoma, along with other brain lesions.
If multiple members of your family have been diagnosed with brain tumors or with cancers associated with these syndromes, genetic counseling can help determine whether an inherited mutation is involved.
Occupational and Chemical Exposures
People who work in farming and agriculture face a higher-than-average risk for brain tumors, likely due to long-term pesticide exposure. A large study found that farmers and farmworkers had roughly 2.5 times the risk of glioma compared to the general population. Women who reported using herbicides had about 2.4 times the risk of meningioma, with risk climbing as years of exposure increased.
The biological explanation may involve certain pesticide ingredients. Some contain compounds that the body converts into nitroso chemicals, which have been linked to nervous system tumors in animal studies. Italian research found a fourfold excess risk of glioma among farmers using insecticides and fungicides, and a threefold excess among herbicide users. Swedish studies have pointed to elevated risk among farm supervisors, forestry workers, and horticultural workers.
Outside of agriculture, some studies have flagged occupational exposure to solvents, vinyl chloride, and rubber manufacturing chemicals, though the evidence is less consistent than it is for pesticides.
Body Weight and Meningioma Risk
Obesity has a notable connection to meningiomas specifically. A meta-analysis pooling data from multiple studies found that people classified as obese had a 48% higher risk of developing meningiomas compared to people at a normal weight. Even being overweight (but not obese) was associated with an 18% increase. For every 5-unit rise in BMI, meningioma risk climbed about 19%.
The connection likely involves hormones. Fat tissue produces estrogen, and meningiomas often have hormone receptors on their surface. This hormonal link may also explain why meningiomas are more common in women than in men. No similar relationship has been found between body weight and gliomas.
Cell Phones and Radiofrequency Energy
Cell phones emit radiofrequency energy, a type of non-ionizing radiation, and the question of whether this contributes to brain cancer has been studied extensively. In 2011, the International Agency for Research on Cancer classified radiofrequency fields as “possibly carcinogenic to humans,” a category that reflects limited and inconclusive evidence rather than a confirmed risk.
Since then, several large studies, including the Interphone study, a Danish cohort study, the UK’s Million Women Study, and the ongoing COSMOS study, have examined the question from different angles. The National Cancer Institute’s summary of this body of evidence is straightforward: the findings are mixed, but overall they do not show an association between cell phone use and cancer. Given that cell phone use has exploded worldwide over the past two decades without a corresponding rise in brain tumor incidence, most researchers consider the risk, if it exists at all, to be very small.
Age and Other Demographic Patterns
Age is one of the strongest predictors of brain tumor type. Glioblastoma, the most aggressive form, peaks in adults over 65. Meningiomas also become more common with age and are about twice as frequent in women. In children, the most common brain tumors are medulloblastomas and pilocytic astrocytomas, which behave very differently from adult brain cancers.
A weakened immune system, whether from HIV, organ transplant medications, or other causes, increases the risk of a specific type called primary central nervous system lymphoma. Beyond that, no dietary pattern, stress level, or common lifestyle habit has been convincingly linked to brain tumor development. For the vast majority of people who develop a brain tumor, there is no preventable cause that can be identified after the fact.