Most brain tumors have no single identifiable cause. They develop when cells in or around the brain accumulate genetic mutations that allow them to grow uncontrollably, but what triggers those mutations is unknown in the majority of cases. Only a handful of risk factors have been firmly established by research, and even those account for a small fraction of diagnoses.
Understanding what is and isn’t linked to brain tumors can help put your risk in perspective.
How Brain Tumors Form
A brain tumor begins when a cell in the brain or its surrounding tissues picks up mutations in genes that control growth. These aren’t usually inherited mutations. They’re acquired over a person’s lifetime, accumulating one after another in a single cell until that cell loses the normal brakes on division. The result is a mass of abnormal tissue that can press on healthy brain structures, disrupt signaling, and cause symptoms.
Different mutations produce different tumor types. Researchers have identified a mutation in a gene called FGFR1, for example, that appears in about 20% of a specific subtype of glioma affecting the brain’s midline. Other well-known genetic changes involve genes responsible for suppressing tumor growth. When those genes stop functioning, cells that would normally be held in check are free to multiply.
Primary Versus Metastatic Tumors
Primary brain tumors originate inside the brain itself. These include gliomas (which arise from the brain’s support cells), meningiomas (which grow in the membranes surrounding the brain), and several rarer types. In children, the most common malignant brain tumors are pilocytic astrocytomas, medulloblastomas, and malignant gliomas, with a median age at diagnosis of 8.
Metastatic brain tumors, by contrast, start as cancer somewhere else in the body and spread to the brain. Virtually any cancer can do this, but some are far more likely to. Melanoma has a particularly strong tendency to reach the brain. Lung cancer is the most common source of brain metastases in men, and breast cancer is the most common source in women. Kidney and colon cancers also frequently spread there. Metastatic brain tumors are actually more common than primary ones in adults.
Confirmed Risk Factors
Ionizing Radiation
The strongest established environmental risk factor for brain tumors is exposure to ionizing radiation. This includes radiation therapy to the head (often given for a previous cancer) and, to a lesser degree, repeated diagnostic imaging. A large European study of children and young adults who received CT scans found a clear dose-response relationship: for every 100 milligray increase in cumulative radiation dose to the brain, the risk of brain cancer roughly doubled. The risk for gliomas specifically showed a similar pattern. This doesn’t mean a single CT scan is dangerous, but it does explain why doctors limit unnecessary imaging, especially in children.
Genetic Syndromes
A small percentage of brain tumors are tied to inherited genetic conditions. Neurofibromatosis types 1 and 2, Li-Fraumeni syndrome, tuberous sclerosis, and Von Hippel-Lindau disease all significantly raise the risk. If you have a close family member with one of these conditions, your own risk profile is different from the general population’s. But these syndromes are rare and explain only a small share of all brain tumor diagnoses.
Factors With Weaker or Mixed Evidence
Chemical Exposures
Certain occupational exposures have been studied extensively, but the evidence is less clear-cut than many people assume. A large hospital-based study examining pesticide exposure and brain cancer found no overall link between insecticide or herbicide use and glioma risk in either men or women. For meningioma, the picture was slightly different: women who reported using herbicides had roughly 2.4 times the risk compared to women who never used them, with the risk increasing alongside years of exposure. No similar association was found in men. Industrial solvents like vinyl chloride have also been investigated, but the data remains inconsistent.
Cell Phones
The question of whether cell phones cause brain tumors has been studied for over two decades. In 2011, the International Agency for Research on Cancer classified radiofrequency radiation from cell phones as “possibly carcinogenic to humans,” which is a cautious category based on limited and inconsistent evidence. The working group acknowledged that the findings from human studies could not be entirely dismissed, but also noted they could reflect chance or study design flaws rather than a real biological effect. Since then, brain tumor rates have not risen in parallel with the explosion in cell phone use, which has made many researchers more skeptical of a meaningful connection.
Factors That May Lower Risk
One of the more surprising findings in brain tumor research involves allergies. A study tracking people over 20 years found that men and women whose blood samples contained allergy-related antibodies had an almost 50% lower risk of developing glioma compared to those without signs of allergies. Among women specifically, elevated allergy markers were associated with a 54% decreased risk of glioblastoma, the most aggressive type. Even a general marker of immune activation (total IgE, a broad antibody measurement) was linked to a 25% lower glioma risk across both sexes.
The leading explanation is that the heightened immune surveillance seen in people with respiratory allergies may help the body detect and destroy abnormal brain cells before they form a tumor. This doesn’t mean allergies are desirable, but it does suggest the immune system plays a meaningful role in brain tumor development.
What Symptoms Look Like
Brain tumors are often diagnosed only after they grow large enough to create pressure inside the skull or interfere with nearby brain tissue. Headaches are the most common symptom, occurring in about half of all people with brain tumors. These headaches often differ from typical tension headaches: they may be worse in the morning, worsen with coughing or bending over, or resist standard pain relievers.
Other common early symptoms include nausea, balance problems, changes in vision or hearing, seizures (sometimes the very first sign), and cognitive changes like difficulty concentrating or personality shifts. The specific symptoms depend heavily on where in the brain the tumor is located. A tumor near the hearing nerve, for instance, may cause hearing loss and balance changes long before a headache develops.
Why Most Cases Have No Clear Cause
For the majority of people diagnosed with a brain tumor, there is no identifiable exposure, no inherited syndrome, and no lifestyle factor that explains it. This is one of the most frustrating realities of the disease. The mutations that drive these tumors appear to arise from the ordinary, unavoidable process of cell division. Every time a cell copies its DNA, there is a small chance of error. Over a lifetime, these random errors can accumulate in the wrong genes, in the wrong cell, and a tumor begins.
This randomness means that brain tumors can affect anyone at any age, regardless of health habits. It also means that in most individual cases, the honest answer to “how did I get this?” is that it was not preventable and not caused by something you did or failed to do.