Most brain tumors have no single identifiable cause. They develop when cells in or around the brain accumulate changes in their DNA that allow them to grow and divide uncontrollably. For some people, a specific risk factor like radiation exposure or an inherited genetic condition is clearly involved. For the majority, the trigger remains unknown, which is one of the most frustrating realities of this diagnosis.
Brain tumors fall into two broad categories: primary tumors that start in the brain itself, and secondary (metastatic) tumors that spread to the brain from cancer elsewhere in the body. The causes and risk factors differ for each.
How Brain Cells Become Tumors
Every cell in your body has built-in controls that regulate when it grows, divides, and dies. Brain tumors form when these controls break down. This typically involves two types of genetic errors: activation of genes that accelerate cell growth (called oncogenes) or the loss of genes that normally act as brakes on cell division (tumor suppressors). Key tumor suppressor genes involved in brain tumors include TP53, PTEN, Rb, and NF1. When one or more of these genes stop working, cells can multiply without restraint and eventually form a mass.
These genetic changes usually happen randomly over a person’s lifetime rather than being inherited. Each time a cell divides, there’s a small chance of a copying error in its DNA. Most errors are harmless or get repaired. Occasionally, an error hits a critical gene, and if enough of these accumulate in the same cell, a tumor can begin. This is partly why brain tumor rates climb with age: more years of cell division means more opportunities for errors. The highest incidence rate, about 20 cases per 100,000 people, occurs in adults 75 and older.
Ionizing Radiation Is the Strongest Known Risk Factor
Exposure to ionizing radiation is the most clearly established environmental cause of brain tumors. This includes radiation therapy to the head (often given for a previous cancer), certain diagnostic imaging, and, historically, exposure from nuclear events. The risk depends heavily on dose and the age at exposure. Data from atomic bomb survivors shows that people exposed at age 10 had a significantly elevated risk of brain cancer later in life, while those exposed at age 40 had a much smaller, statistically uncertain increase. Children’s developing brains are far more vulnerable to radiation-induced DNA damage.
For most people today, the relevant concern is medical radiation. Radiation therapy directed at the head, particularly during childhood, carries a measurable long-term risk. Standard diagnostic X-rays deliver very low doses, and CT scans deliver moderate doses. Some studies have suggested a link between CT scans in childhood and later brain tumor risk, though researchers have debated whether this finding reflects true causation or is partly explained by the fact that children who already have early symptoms are more likely to receive CT scans in the first place.
Inherited Genetic Conditions
A small percentage of brain tumors are linked to inherited syndromes that run in families. These conditions don’t guarantee a tumor will develop, but they raise the odds substantially.
- Neurofibromatosis type 1 (NF1) is one of the most common. About 15% of people with NF1 develop tumors along the optic nerve pathway, usually before age 7. They also have a roughly 50-fold higher risk of aggressive brain tumors compared to the general population.
- Neurofibromatosis type 2 (NF2) primarily causes tumors on the nerves responsible for hearing and balance, as well as meningiomas, which grow from the membranes surrounding the brain.
- Li-Fraumeni syndrome results from inherited mutations in the TP53 gene. Brain tumors appear in about 14% of people carrying these mutations, with an average onset around age 16. The tumor types range from aggressive gliomas in young adults to rarer tumors in younger children.
- Turcot syndrome / Lynch syndrome involves inherited defects in DNA repair genes. People with Lynch syndrome have roughly a 3% lifetime risk of developing a brain tumor, most often glioblastoma.
If multiple family members have had brain tumors or cancers associated with these syndromes, genetic testing can help clarify whether an inherited mutation is involved.
Metastatic Tumors: Cancer That Spreads to the Brain
Secondary brain tumors, those that spread from a cancer originating elsewhere, are actually more common than primary brain tumors in adults. The cancer cells travel through the bloodstream and cross into brain tissue.
Lung cancer is by far the most common source, accounting for at least half of all brain metastases. Breast cancer follows at 15 to 25%, then melanoma at 5 to 20%. Kidney, colon, and thyroid cancers also spread to the brain, though less frequently. In principle, any type of cancer can metastasize there. The blood-brain barrier, a protective layer that normally restricts what enters the brain, can be disrupted by tumor cells, allowing them to establish new growths in brain tissue.
Weakened Immune System
A suppressed immune system is a strong risk factor for one specific type of brain tumor: primary central nervous system lymphoma (PCNSL). People living with HIV and organ transplant recipients taking anti-rejection medications have a dramatically higher risk of this cancer compared to the general population. In immunosuppressed individuals, Epstein-Barr virus infection plays a role in driving the development of these lymphomas. Among people with healthy immune systems, PCNSL linked to Epstein-Barr virus is uncommon.
Occupational and Chemical Exposures
Researchers have investigated whether workplace chemicals increase brain tumor risk, with pesticides receiving the most attention. A large hospital-based study of glioma and meningioma found no overall link between insecticide exposure and brain tumors in either men or women. However, women with long-term occupational exposure to herbicides had a significantly increased risk of meningioma, with risk rising as years of exposure increased. Some pesticides contain compounds that the body can convert into cancer-promoting chemicals known to cause nervous system tumors in animal studies.
Other industrial chemicals, including vinyl chloride and certain solvents, have been investigated with mixed results. No single workplace chemical has been established as a definitive cause of brain tumors in the way that ionizing radiation has, but the herbicide finding in women suggests that prolonged chemical exposure deserves continued scrutiny.
Cell Phones and Radiofrequency Energy
The International Agency for Research on Cancer (IARC), part of the World Health Organization, classified radiofrequency electromagnetic fields as “possibly carcinogenic to humans” in 2011. This classification (Group 2B) was based on limited evidence of an association between heavy wireless phone use and glioma. One study included in the evaluation found a 40% increased risk of glioma among the heaviest cell phone users, defined as an average of 30 minutes per day over 10 years.
“Possibly carcinogenic” is a cautious classification that sits well below “probably” or “confirmed” carcinogenic. It means the evidence hints at a connection but isn’t strong enough to draw firm conclusions. The IARC described the evidence for glioma and acoustic neuroma as limited, and for all other cancer types as inadequate. Large-scale studies since then have not consistently confirmed an increased risk, and brain tumor rates have not risen in parallel with the massive global increase in cell phone use over the past two decades.
Processed Meat and Diet
You may have seen claims that processed meats cause brain tumors because they contain compounds called nitrosamines, which are potent carcinogens in animal studies. However, a large prospective study tracking dietary habits and glioma risk found no association. People who ate the most processed meat had no higher glioma risk than those who ate the least. Intake of nitrates, nitrites, and specific nitrosamines also showed no connection to brain tumor risk. While nitrosamines can cause brain tumors when given directly to lab animals, the amounts people consume through food do not appear to translate into measurable risk.
Age, Sex, and Demographics
Brain tumors can occur at any age, but risk increases significantly with each decade of life. In U.S. data spanning 1999 to 2020, the rate per 100,000 people roughly doubles with each age bracket: from about 3.1 in children under 15 to 6.4 in people aged 45 to 54, then 17.2 in the 65 to 74 range. Men are diagnosed more often than women, making up about 56% of cases. This sex difference is consistent across studies worldwide, though the reasons behind it remain unclear. Interestingly, meningiomas are an exception, occurring more frequently in women.
Recent trends show a notable increase in incidence among people under 35, while rates for those 35 and older have been stable or slightly declining. Whether the rise in younger adults reflects better detection through improved imaging or a true increase in tumors is still being studied.