A low-grade glioma is a slow-growing brain tumor that arises from glial cells, the supportive tissue of the brain. These tumors are classified as grade 1 or grade 2 by the World Health Organization, meaning they grow more slowly and are less aggressive than their grade 3 or grade 4 counterparts. Despite being called “low grade,” these tumors are serious. About 72% of grade 2 gliomas eventually transform into higher-grade tumors over time, making early detection and treatment decisions critical.
Types of Low-Grade Glioma
The WHO’s 2021 classification system now defines gliomas primarily by their molecular features rather than just how they look under a microscope. For adult-type low-grade gliomas (grade 2), there are two main categories. The first is astrocytoma, which carries a mutation in the IDH gene but does not have a specific chromosomal change called 1p/19q codeletion. The second is oligodendroglioma, which has both the IDH mutation and 1p/19q codeletion. These molecular distinctions matter because they significantly influence how the tumor behaves and responds to treatment.
Grade 1 gliomas are a separate group entirely and tend to occur in children and young adults. The most common is pilocytic astrocytoma, which is often well-contained and curable with surgery alone. These tumors carry different genetic signatures, typically involving changes in a signaling pathway called MAPK rather than IDH mutations.
There’s also an important category to be aware of: some tumors that look low-grade under the microscope turn out to lack the IDH mutation entirely (called IDH-wildtype). When additional molecular testing reveals features resembling glioblastoma, such as specific chromosomal gains and losses, these tumors behave aggressively regardless of their low-grade appearance and require immediate, intensive treatment.
How Low-Grade Gliomas Are Found
Seizures are the most common reason a low-grade glioma is discovered. About 35% of patients experience seizures at diagnosis, while 36% show cognitive changes like difficulty with memory, attention, or word-finding. Headaches bring roughly 31% of patients to medical attention, and about 22% notice weakness or coordination problems. Some tumors are found incidentally on brain scans done for unrelated reasons.
MRI is the primary tool for identifying these tumors. Low-grade gliomas typically appear as bright areas on certain MRI sequences (T2-weighted images) and, importantly, they usually do not light up when contrast dye is injected. Contrast enhancement on MRI often signals a higher-grade tumor or one that is starting to transform. One particularly useful MRI pattern, called the T2-FLAIR mismatch sign, involves a specific combination of brightness on one sequence and relative darkness on another, with a bright rim around the edges. When this pattern is present, it identifies an IDH-mutant astrocytoma with 100% accuracy. While only about half of IDH-mutant astrocytomas show this sign, its presence gives doctors confident molecular information before any tissue is even obtained.
Why Molecular Subtype Matters
The IDH mutation is central to how low-grade gliomas are understood today. The mutant form of the IDH enzyme produces an abnormal metabolite that accumulates in tumor tissue and disrupts normal cell processes, including gene regulation and cell maturation. Paradoxically, tumors with this mutation tend to grow more slowly and respond better to treatment than those without it.
The 1p/19q codeletion, a loss of two specific chromosome arms found in oligodendrogliomas, further refines the prognosis. Oligodendrogliomas with both the IDH mutation and 1p/19q codeletion generally have the most favorable outcomes among grade 2 gliomas and respond particularly well to chemotherapy. IDH-mutant astrocytomas (without the codeletion) have a somewhat less favorable but still relatively good outlook. IDH-wildtype tumors that end up reclassified as more aggressive carry the worst prognosis.
Surgery and Its Impact on Survival
Surgery is the first-line treatment for most low-grade gliomas, and the amount of tumor removed makes a measurable difference in survival. Across all molecular subtypes combined, patients whose surgeons removed a larger percentage of tumor had significantly better overall survival compared to those with less complete resections. This effect was especially pronounced in oligodendrogliomas: the survival benefit of a more complete resection was over six times greater when comparing patients in the top versus bottom quartile of tumor removal. IDH-mutant astrocytomas showed a similarly strong benefit.
Interestingly, for the small subset of IDH-wildtype lower-grade gliomas, the extent of surgical removal did not appear to influence overall survival, likely because these tumors behave aggressively regardless. This finding reinforces why molecular testing is so important: it changes not only prognosis but also how much surgical risk is worth taking.
Because many low-grade gliomas grow in or near areas of the brain that control speech, movement, or other critical functions, surgeons often use specialized techniques like awake craniotomy and real-time brain mapping to remove as much tumor as possible while preserving function.
Radiation, Chemotherapy, and Newer Treatments
After surgery, the decision about whether to add radiation and chemotherapy depends on risk factors. A landmark clinical trial established that patients older than 40 or those who had an incomplete surgical resection benefit from immediate radiation and chemotherapy after surgery. Patients younger than 40 with a complete resection are often monitored with regular MRI scans instead, an approach sometimes called watchful waiting or active surveillance. Patients with seizures that don’t respond well to medication may also benefit from starting treatment sooner rather than waiting.
A major advance arrived in 2023 with vorasidenib, the first oral medication designed specifically for IDH-mutant low-grade gliomas. This drug crosses the blood-brain barrier and blocks the mutant IDH enzyme that drives tumor growth. In a clinical trial published in the New England Journal of Medicine, patients taking vorasidenib had a median time before their disease worsened of 27.7 months, compared to 11.1 months for those on placebo. It also delayed the need for additional treatments like radiation or further surgery. This represents a meaningful new option, particularly for patients whose tumors are being monitored or have started to grow again.
Malignant Transformation
One of the most important things to understand about grade 2 gliomas is that they are not static. Historical data shows that roughly 72% of these tumors will eventually transform into higher-grade, more aggressive tumors. However, the timeline varies considerably. Five-year freedom from malignant transformation is estimated at about 86%, meaning most patients have several years before this shift occurs, if it does. The molecular subtype again plays a role: IDH-mutant tumors with 1p/19q codeletion tend to have longer periods of stability than other subtypes.
This tendency toward transformation is a key reason why ongoing monitoring with regular MRI scans is essential, even after successful surgery. New contrast enhancement, rapid growth, or worsening symptoms can all signal that a tumor is becoming more aggressive.
Living With a Low-Grade Glioma
Because many people with low-grade gliomas live for years or even decades, quality of life becomes a central concern. Cognitive changes are among the most common and persistent challenges, affecting attention, processing speed, memory, and language. These deficits can be subtle enough to escape detection on formal neuropsychological tests yet still interfere significantly with daily life. Roughly half of glioma survivors in one study were not employed, and the majority of those who stopped working attributed it directly to the impact of the tumor on their lives.
Research suggests that a person’s own perception of their cognitive abilities, along with mood, physical well-being, and social connection, are actually stronger predictors of daily functioning than objective test scores. Depression, anxiety, loneliness, and sleep problems all compound the effects of cognitive changes. This means that support for glioma survivors needs to go beyond treating the tumor itself. Cognitive rehabilitation, psychological support, and attention to sleep and physical health all play a role in helping people maintain their daily routines, relationships, and work capacity over the long course of this disease.