Brain tumors develop when cells in or around the brain accumulate genetic changes that override the normal controls on cell growth and division. About 6.1 out of every 100,000 people are diagnosed with brain or nervous system cancer each year in the United States. Some tumors originate in brain tissue itself, while others spread to the brain from cancers elsewhere in the body. The path from a single malfunctioning cell to a detectable tumor involves a chain of biological events, each building on the last.
How Normal Brain Cells Lose Control
Every cell in your body carries built-in instructions that tell it when to grow, when to stop growing, and when to die. Brain tumors begin when mutations disable these instructions. Two categories of genes matter most: genes that promote cell growth (oncogenes) and genes that act as brakes on growth (tumor suppressor genes). When mutations flip growth-promoting genes into overdrive or knock out the braking genes, cells start dividing without the usual checkpoints.
In glioblastoma, the most aggressive primary brain cancer, tumor cells often carry many extra copies of a growth-promoting gene called EGFR. Researchers at the National Cancer Institute found that some glioblastoma cells store these extra gene copies on small circles of DNA that float separately from the cell’s normal chromosomes. This is a significant detail because those free-floating DNA circles can increase or decrease the number of gene copies in response to treatment, helping tumor cells dodge drugs designed to block that gene. When scientists used gene-editing techniques to disable the molecular “on switches” near these extra copies, both gene activity and cell survival dropped, confirming that the amplified gene is directly fueling tumor growth.
Mutations in the well-known tumor suppressor gene TP53 are also common across many brain tumor types. When TP53 stops working, cells that would normally be flagged and destroyed instead survive and continue dividing. Over time, additional mutations accumulate, and the cell population becomes increasingly abnormal.
How Tumors Build Their Own Blood Supply
A cluster of rogue cells can only grow so large before it needs its own dedicated blood supply. Brain tumors solve this problem by releasing chemical signals that recruit new blood vessels, a process called angiogenesis. The primary signal is a protein called VEGF, which binds to receptors on nearby blood vessel cells and triggers them to multiply, migrate toward the tumor, and form new vessel branches.
Research published in the journal Cancer Research found that a specific population of stem-like cells within gliomas produces 10 to 20 times more VEGF than the rest of the tumor cells. Low oxygen levels inside the growing tumor mass push these cells to release even more of the signal. In laboratory models, the fluid surrounding these stem-like cells dramatically increased the growth and organization of blood vessel cells compared to fluid from ordinary tumor cells. Blocking VEGF with the antibody drug bevacizumab specifically shut down these effects, which is why that drug is now used clinically to starve certain brain tumors of their blood supply.
This ability to build a vascular network is one of the key features that separates aggressive tumors from slower-growing ones. Without robust angiogenesis, a tumor remains small and relatively contained.
Slow-Growing Tumors Can Turn Aggressive
Not all brain tumors start out dangerous. Low-grade gliomas (classified as grade 2) grow slowly, sometimes for years, before they transform into higher-grade, more aggressive cancers. A large Cleveland Clinic study tracking 486 patients with low-grade gliomas found that malignant transformation occurred in about 84 patients over a median follow-up of 5.3 years. At the five-year mark, roughly 86% of patients had not yet experienced this shift.
Several factors increased the risk of transformation. Tumors 5 centimeters or larger were more likely to become aggressive. Male patients had an independently higher risk even after accounting for molecular differences. Certain molecular profiles of the tumor itself also played a role. The transformation typically involves the tumor acquiring additional mutations that accelerate growth, increase blood vessel recruitment, and allow cells to invade surrounding healthy brain tissue more readily.
This progression explains why even a “benign” or low-grade brain tumor diagnosis requires ongoing monitoring. The biology can change.
When Cancer Spreads to the Brain From Elsewhere
Roughly half of all brain tumors are not primary brain cancers. They are metastatic tumors: colonies established by cancer cells that traveled from the lungs, breast, skin, or other organs. The journey these cells take is remarkably complex.
First, cancer cells from the original tumor break free and enter the bloodstream. To survive in circulation, they coat themselves with a shield made of fibrin and platelets, the same clotting proteins your blood uses to seal wounds. This disguise helps them avoid detection by the immune system. The traveling cells also produce sticky molecules that let them slow down and latch onto blood vessel walls, much like a boat tying up at a dock.
Reaching the brain presents an extra challenge: the blood-brain barrier, a tightly sealed layer of cells lining the brain’s blood vessels that normally blocks most substances from entering brain tissue. Tumor cells break through by releasing enzymes (metalloproteinases and cathepsins) that dissolve the structural proteins holding the barrier together. Ironically, some cancer treatments can temporarily weaken the blood-brain barrier as well, inadvertently giving circulating cancer cells an easier entry point. Once inside brain tissue, the cells establish a new blood supply using the same VEGF-driven process that primary tumors use, and a secondary tumor begins to grow.
Genetic Conditions That Raise Risk
Most brain tumors arise from random or environmentally triggered mutations, but some people carry inherited genetic conditions that significantly increase their risk. Neurofibromatosis type 1 (NF1) is one of the most studied. In a large study of NF1 patients conducted by the National Cancer Institute, glioma was the most common cancer diagnosed, affecting about 18% of patients in the study group. NF1 disables a gene that normally restrains cell growth, leaving brain cells more vulnerable to tumor formation from an early age.
Li-Fraumeni syndrome, caused by inherited mutations in the TP53 tumor suppressor gene, also raises the lifetime risk of brain tumors substantially, along with the risk of many other cancer types. Because every cell already carries one defective copy of TP53, it takes only one additional mutation to knock out the gene entirely.
These hereditary syndromes account for a small fraction of all brain tumors, but they illustrate how central tumor suppressor genes are to keeping brain cell growth in check.
Environmental Risk Factors
Ionizing radiation is the most clearly established environmental risk factor for brain tumors. People who received radiation therapy to the head, particularly during childhood, face an elevated risk years or even decades later. The National Cancer Institute is currently conducting a large pooled analysis of 16 epidemiological studies to better quantify radiation-related brain tumor risk, with a particular focus on CT scans. The growing use of CT imaging in medical care has made this a significant public health question, since each scan delivers a small dose of radiation to the brain.
Beyond radiation, no other environmental exposure has been conclusively linked to brain tumors in large-scale studies, though research into chemical exposures and electromagnetic fields continues.
How Location Shapes Symptoms
Brain tumors produce symptoms not only because of uncontrolled cell growth but because that growth physically disrupts the brain region where it occurs. A tumor in the temporal lobe, located near your temples, can cause memory problems or hallucinations involving sight, taste, or smell. Frontal lobe tumors often affect personality, decision-making, and movement. Tumors near the back of the brain can interfere with vision.
Many symptoms come not from the tumor cells themselves but from the pressure they create. As a tumor grows inside the rigid skull, it compresses surrounding tissue and can block the normal flow of cerebrospinal fluid. This raises pressure inside the skull, leading to headaches, nausea, and vision changes that may be the first signs something is wrong. Slower-growing tumors give the brain more time to compensate, which is why they can reach a surprising size before causing noticeable symptoms.