A glial tumor, commonly called a glioma, is a type of brain or spinal cord tumor that grows from glial cells, the supportive cells that surround and protect neurons in the central nervous system. Gliomas account for a significant portion of all primary brain tumors and range widely in severity, from slow-growing tumors that may be managed for years to aggressive cancers like glioblastoma. Brain and nervous system cancers overall represent about 1.2% of all new cancer cases in the United States, with an incidence rate of roughly 6.1 per 100,000 people per year.
Where Glial Tumors Come From
Your brain contains billions of neurons, but it also contains billions of glial cells that perform essential housekeeping functions. The three main types of glial cells each give rise to a different category of tumor.
Astrocytes are star-shaped cells with thousands of branching processes that touch nearly every other cell type in the brain. They help form and maintain the connections between neurons, wrap around blood vessels to regulate nutrient delivery, and keep the brain’s chemical environment stable. When these cells become cancerous, the result is an astrocytoma.
Oligodendrocytes produce the fatty myelin sheaths that wrap around nerve fibers and speed up electrical signals. Tumors arising from these cells are called oligodendrogliomas. Ependymal cells line the fluid-filled ventricles of the brain and spinal cord, helping to produce and circulate cerebrospinal fluid. Tumors from these cells are ependymomas.
How Gliomas Are Classified
The World Health Organization updated its classification system for brain tumors in 2021, making molecular markers (specific genetic changes inside tumor cells) just as important as how the tumor looks under a microscope. For the first time, adult and pediatric gliomas are now classified separately because they develop through different genetic pathways and carry different prognoses.
Adult-type diffuse gliomas, the most common category, are divided into three types based on their molecular profile:
- Astrocytoma, IDH-mutant: carries a specific genetic change (IDH mutation) that generally signals a better outlook than tumors without it.
- Oligodendroglioma, IDH-mutant with 1p/19q codeletion: has both the IDH mutation and the loss of two chromosome segments, a combination linked to slower growth and better treatment response.
- Glioblastoma, IDH wild-type: lacks the IDH mutation entirely and is the most aggressive form.
This molecular approach matters because two tumors that look identical under a microscope can behave very differently depending on which genetic changes they carry. Research has shown that molecular classification more accurately predicts outcomes than the older method of basing a diagnosis on cell appearance alone.
Grades and What They Mean
Gliomas are graded from 1 to 4. Grade 1 tumors are typically slow-growing and well-defined, often curable with surgery alone. Grade 2 tumors grow slowly but tend to infiltrate surrounding brain tissue. Grade 3 tumors are more actively growing and show abnormal cell features. Grade 4 is the most aggressive, with glioblastoma being the best-known example.
A critical detail in the 2021 classification: a tumor that was previously called an “IDH-mutant glioblastoma” is now reclassified as an astrocytoma, IDH-mutant, grade 4. The term “glioblastoma” is reserved exclusively for tumors without the IDH mutation. This distinction reflects real differences in how patients respond to treatment and how long they survive.
Common Symptoms
Symptoms depend heavily on where in the brain the tumor develops, but some patterns are consistent. About half of people with a glioma experience headaches, which in glioblastoma cases tend to be severe and worst in the morning. Seizures are one of the earliest signs, particularly with astrocytomas.
Other symptoms include nausea and vomiting from increased pressure inside the skull, confusion, memory problems, difficulty speaking, weakness on one side of the body, balance issues, and restricted field of vision. Because these symptoms overlap with many other conditions, they often don’t immediately point to a brain tumor, which can delay diagnosis.
How Gliomas Are Diagnosed
MRI is the primary imaging tool. Standard MRI scans reveal the tumor’s location, size, and relationship to surrounding structures. A specialized technique called magnetic resonance spectroscopy can be added to analyze the chemical composition of the tissue, helping distinguish tumor from other types of brain lesions with about 88% sensitivity. MRS can also help guide where a surgeon should take a tissue sample by identifying the areas with the highest tumor activity.
Despite advances in imaging, a tissue sample remains essential. A biopsy or surgical removal provides the material needed for both microscopic examination and molecular testing, which together determine the tumor’s exact type and grade. This information directly shapes the treatment plan.
Risk Factors
Moderate-to-high-dose ionizing radiation is the only firmly established environmental risk factor for brain tumors. This includes prior radiation therapy to the head, particularly when the exposure occurred at a younger age. Certain inherited genetic syndromes also increase risk, though they account for a small minority of cases. For most people diagnosed with a glioma, no clear cause is identified.
Treatment Approaches
The first step for most gliomas is surgery. The goal is maximal safe resection, meaning the surgeon removes as much tumor as possible while preserving neurological function. This serves a dual purpose: it reduces the tumor burden and provides tissue for a definitive diagnosis. Greater extent of removal is associated with better functional outcomes and longer survival.
For higher-grade gliomas, surgery alone is not sufficient. The standard follow-up is radiation therapy combined with chemotherapy. The chemotherapy drug temozolomide is the most widely used agent for glioblastoma. In clinical trials, adding temozolomide to radiation improved five-year survival from 2% to 10% and one-year survival from 11% to 27% compared to radiation alone. While those numbers are still sobering, the combination remains the backbone of glioblastoma treatment.
For patients in poor overall health, radiation with supportive care may be the primary approach rather than aggressive combination therapy.
Tumor Treating Fields
A newer treatment option for glioblastoma involves a wearable device that delivers low-intensity alternating electrical fields to the scalp. These fields interfere with the ability of rapidly dividing tumor cells to complete cell division. The device, approved by the FDA for newly diagnosed glioblastoma in 2015, is worn for most of the day and used alongside standard chemotherapy.
In clinical trials, adding this therapy to temozolomide increased median overall survival by roughly five months and reduced the risk of death by 30 to 40% compared to standard treatment alone. Side effects are primarily skin irritation at the electrode sites. Results in recurrent glioblastoma have been more mixed, with some studies showing benefit in patients whose tumors carry specific genetic changes while others show limited improvement.
Survival and Prognosis
Prognosis varies enormously depending on tumor type, grade, molecular markers, and age at diagnosis. Low-grade gliomas (grade 2) have a mean survival of about seven years, with younger patients faring better. Among low-grade subtypes, oligodendrogliomas carry the longest median survival at 7.2 years, compared to 5.6 years for mixed gliomas and 5.2 years for astrocytomas. About 20% of low-grade glioma patients survive at least two decades.
The challenge with low-grade gliomas is that virtually all of them eventually transform into higher-grade tumors over time. This progression is what ultimately drives the disease, making long-term monitoring essential even when the initial tumor appears slow-growing.
Glioblastoma has the poorest prognosis of any glioma. Even with the best available treatment, most patients survive between 15 and 20 months after diagnosis. The molecular profile of the tumor is one of the strongest predictors of outcome. Tumors with the IDH mutation, even when they reach grade 4, carry a meaningfully better prognosis than IDH wild-type glioblastoma.