Most brain cancers have no single, identifiable cause. They result from genetic mutations that cause brain cells to grow and divide uncontrollably, but what triggers those mutations in the first place remains unknown in the majority of cases. The only firmly established environmental cause is exposure to moderate or high doses of ionizing radiation. Everything else, from inherited syndromes to chemical exposures to diet, either accounts for a small fraction of cases or is still under investigation.
How Brain Tumors Start at the Cellular Level
A brain tumor begins when DNA inside a brain cell changes in a way that disables the normal checks on cell growth. Instead of dividing at a controlled pace, the altered cell multiplies without stopping, eventually forming a mass. The specific mutations vary by tumor type, but a few genetic changes show up repeatedly.
One of the most common involves an enzyme called IDH, which helps regulate essential chemical reactions inside cells. When the gene for this enzyme mutates, it disrupts the cell’s metabolism so severely that a normal cell would die. But tumor cells develop a workaround: they activate a chain of molecular signals that protect them from the metabolic stress and allow them to keep multiplying. Over 80 percent of lower-grade gliomas, one of the most common types of brain cancer, carry this IDH mutation.
Other well-known mutations affect genes that normally act as brakes on cell growth (like TP53) or genes that signal cells to divide (like EGFR). When these genes malfunction, the balance between cell growth and cell death tips sharply toward unchecked growth. These mutations can occur randomly during a person’s lifetime as cells divide, which is one reason brain cancer often seems to strike without warning.
Ionizing Radiation: The One Confirmed Environmental Cause
Moderate to high doses of ionizing radiation are the only environmental factor definitively linked to brain tumors. The evidence comes from studies of atomic bomb survivors and children who received radiation therapy for other medical conditions, including radiation to the scalp for fungal infections (a common treatment decades ago) and radiation for childhood cancers.
The gap between radiation exposure and a brain tumor diagnosis can be long. In studies of childhood cancer survivors who received radiation, the risk of a brain tumor was modest in the first 10 to 14 years but roughly doubled at 15 years or more after exposure. Among atomic bomb survivors, the average age at diagnosis was over 60, often decades after the original exposure. This long latency period makes radiation-related brain tumors difficult to predict on an individual level.
Routine diagnostic imaging like a single CT scan or dental X-ray delivers far lower doses than the levels studied in these populations. The established risk applies to repeated or high-dose therapeutic radiation, not everyday medical scans.
Inherited Genetic Syndromes
A small but real percentage of brain tumors trace back to inherited conditions. About 1 to 2 percent of adult gliomas and roughly 4 percent of pediatric brain tumors are linked to known genetic syndromes. The most significant ones include:
- Li-Fraumeni syndrome: Caused by an inherited mutation in the TP53 gene, this condition sharply increases the risk of several cancers, including glioblastoma, the most aggressive type of brain tumor.
- Neurofibromatosis type 1 (NF1): This condition causes benign nerve tumors throughout the body and raises the risk of certain brain tumors, particularly optic nerve gliomas in children.
- Neurofibromatosis type 2 (NF2): Linked to acoustic neuromas (tumors on the hearing nerve), meningiomas, and other central nervous system tumors.
- Turcot syndrome: Associated with colon polyps and an elevated risk of brain tumors, including medulloblastoma and glioma.
All of these syndromes are rare. Having a close family member with a brain tumor does not necessarily mean you carry one of these conditions, and most people diagnosed with brain cancer have no family history of it.
When Cancer Spreads to the Brain From Elsewhere
Not all brain tumors start in the brain. Metastatic brain tumors originate in another organ and travel to the brain through the bloodstream. About 30 percent of cancers found in the brain began as lung, breast, or skin cancer (particularly melanoma). Because the brain has no lymphatic drainage system, blood vessels are the primary route cancer cells use to reach it.
The brain is normally protected by the blood-brain barrier, a tightly sealed lining of blood vessels that keeps most substances out. But metastatic cancer cells can arrest within those vessels, break through the barrier, and use the surrounding blood vessel walls as pathways to migrate deeper into brain tissue. Metastatic brain tumors are actually more common than primary brain tumors and are treated differently, since the underlying cause is the original cancer rather than a problem that started in the brain itself.
Weakened Immune Systems
People with suppressed immune systems face a higher risk of certain brain cancers, particularly a type called primary central nervous system lymphoma. This applies mainly to two groups: organ transplant recipients taking immunosuppressive medications and people living with HIV/AIDS. In both cases, the immune system becomes less capable of detecting and destroying abnormal cells. Infections that a healthy immune system would control, such as Epstein-Barr virus, can also contribute to lymphoma development when immunity is compromised.
Chemical and Occupational Exposures
Several workplace chemicals have been investigated as potential contributors to brain cancer, though the evidence is less definitive than for radiation. A recent meta-analysis of occupational benzene exposure found a 21 percent increase in the risk of nervous system cancers among exposed workers overall, with a particularly strong association for glioma specifically. Benzene is widely used in the petrochemical industry, in rubber manufacturing, and as an industrial solvent.
Other chemicals under study include formaldehyde and vinyl chloride, though findings have been inconsistent across studies. The challenge with occupational research is that workers are often exposed to multiple chemicals simultaneously, making it difficult to isolate the effect of any single substance.
Diet and N-Nitroso Compounds
One dietary factor that has drawn consistent scientific attention is nitrite, a preservative found in cured and processed meats like bacon, hot dogs, and deli meats. Nitrite can be converted in the digestive tract into N-nitroso compounds, which have been detected in brain tissue and are considered one of the more plausible dietary contributors to glioma.
A large meta-analysis found that people with the highest nitrite intake had a 26 percent greater risk of glioma compared to those with the lowest intake. A dose-response analysis estimated that each additional 1 milligram per day of nitrite increased glioma risk by 14 percent. One cohort study tracking over 545,000 people found that higher dietary nitrite was associated with a 32 percent increased risk. For children, dietary nitrate (a related compound found in some water sources and vegetables) showed a link to brain tumors as well.
These findings are observational, meaning they show a statistical association rather than proof of causation. But the consistency across multiple studies and the known biological mechanism (nitrite converting to cancer-promoting compounds in the body) make this one of the more credible dietary risk factors identified so far.
Cell Phones and Brain Cancer
The International Agency for Research on Cancer classified radiofrequency waves from cell phones as “possibly carcinogenic” in 2011, placing them in a broad category that signals limited evidence rather than a confirmed risk. The evidence was considered limited for glioma and acoustic neuroma and inadequate for all other cancer types. Since then, a systematic review of 63 studies commissioned by the World Health Organization concluded with moderate certainty that mobile phone use does not increase cancer risk. Brain tumor incidence rates in the U.S., tracked through national cancer registries, have not shown the sharp increase that would be expected if cell phones were a significant cause, despite decades of widespread use.
How Common Brain Cancer Is
Primary brain and nervous system cancers are relatively uncommon compared to cancers of the lung, breast, or colon. The incidence rate in the United States is 6.1 new cases per 100,000 people per year, based on data from 2018 to 2022. The rarity of the disease is one reason identifying causes has been so difficult: with fewer cases to study, it takes very large populations and long follow-up periods to detect meaningful patterns. For most people who develop a brain tumor, no specific cause is ever identified, which remains one of the most frustrating realities of the disease for patients and researchers alike.