Brain tumors are most commonly grouped into four broad categories: gliomas, meningiomas, pituitary tumors, and metastatic (secondary) tumors. This is a simplified framework, since there are actually dozens of distinct brain tumor types classified by the World Health Organization. But these four account for the vast majority of cases and represent fundamentally different origins, behaviors, and outlooks.
Gliomas: Tumors From Support Cells
Gliomas develop from glial cells, the support cells that surround and protect neurons throughout the brain and spinal cord. They’re the most common type of primary brain tumor, meaning they originate in the brain itself rather than spreading from elsewhere. Gliomas are further divided into subtypes based on which glial cell they arise from.
Astrocytomas start in star-shaped cells called astrocytes. They range from slow-growing, low-grade tumors to glioblastoma, the most aggressive form. Glioblastoma is a grade IV astrocytoma and one of the most difficult brain tumors to treat, growing rapidly and often infiltrating surrounding tissue.
Oligodendrogliomas develop from cells that produce the protective coating around nerve fibers. These tend to grow more slowly and are less common than astrocytomas. They generally respond better to treatment.
Ependymomas arise from cells lining the fluid-filled cavities inside the brain and spinal cord. These are rare and can occur at any age, though they’re more common in children.
All gliomas are graded on a scale from 1 to 4, with grades 1 and 2 considered low-grade (slower growing) and grades 3 and 4 considered high-grade (more aggressive). A pathologist determines the grade by examining tissue under a microscope, looking for signs of rapid cell division, abnormal blood vessel growth, and dead tissue. Low-grade gliomas can progress to higher grades over time.
Meningiomas: Tumors of the Brain’s Lining
Meningiomas grow from cells in the meninges, the protective membranes that wrap around the brain and spinal cord. They account for roughly 13% to 26% of all tumors inside the skull. Unlike gliomas, meningiomas typically grow on the surface of the brain rather than within it, pressing inward as they enlarge.
Most meningiomas are slow-growing and benign, meaning they don’t invade surrounding brain tissue. Many are discovered incidentally during imaging for an unrelated issue. Small, asymptomatic meningiomas are often monitored with periodic scans rather than treated immediately. When they do cause symptoms, it’s usually because they’ve grown large enough to put pressure on nearby brain structures, which can cause headaches, vision changes, or weakness depending on their location.
A small percentage of meningiomas are atypical or malignant and grow more aggressively. Genetic changes on chromosome 22 are commonly found in meningioma cells and play a role in their development.
Pituitary Tumors
Pituitary tumors, most commonly pituitary adenomas, develop in the pea-sized gland at the base of the brain that controls hormone production. The vast majority are benign and slow-growing. They fall into two functional categories: those that produce excess hormones (“functioning” adenomas) and those that don’t (“nonfunctioning” adenomas).
Functioning adenomas cause problems by flooding the body with too much of a particular hormone. The most common type overproduces prolactin, a hormone involved in milk production, which can cause irregular periods, unexpected breast milk production, or fertility issues. Other functioning adenomas produce excess growth hormone or cortisol, each causing its own distinct set of symptoms.
Nonfunctioning adenomas don’t produce hormones but can still cause trouble as they grow. Because the pituitary gland sits just below the point where the optic nerves cross, an enlarging tumor in this area often affects vision first, typically causing loss of peripheral vision on both sides. Larger tumors can also compress the normal pituitary gland enough to reduce its hormone output, leading to fatigue, weight changes, and other hormonal imbalances.
Metastatic Brain Tumors
Metastatic tumors, also called secondary brain tumors, don’t originate in the brain. They start as cancer somewhere else in the body and spread to the brain through the bloodstream. In adults, metastatic tumors are actually more common than primary brain tumors.
The cancers that most frequently spread to the brain are lung, breast, colon, kidney, and melanoma. Less commonly, cancers of the liver, ovary, thyroid, pancreas, and uterus can also reach the brain, as can certain blood cancers. A person can develop a single metastatic tumor or multiple ones scattered through different areas of the brain.
Because metastatic tumors are extensions of a cancer that started elsewhere, treatment depends heavily on the original cancer type and how widespread it has become. The outlook varies enormously based on these factors.
How Location Shapes Symptoms
Regardless of tumor type, symptoms often depend more on where the tumor sits than what kind of cells it contains. The brain is organized into specialized regions, and a tumor disrupts whatever function that region handles.
Tumors in the frontal lobe, the area behind your forehead, tend to cause personality changes, forgetfulness, and loss of interest in usual activities. Family members often notice these shifts before the person does. Temporal lobe tumors (near the temples) can cause memory problems and unusual sensory experiences like seeing, tasting, or smelling things that aren’t there. Parietal lobe tumors, in the upper-middle part of the brain, affect sensory processing and can cause vision or hearing problems. Tumors in the occipital lobe, at the back of the head, primarily cause vision loss.
Some symptoms are common across most brain tumor types and locations. Headaches that worsen over time, especially in the morning, seizures in someone who has never had them, nausea, and progressive weakness or numbness on one side of the body are all warning signs. These symptoms don’t always mean a tumor is present, but they warrant imaging.
How Brain Tumors Are Identified
MRI is the primary tool for detecting and evaluating brain tumors. Standard MRI scans reveal the tumor’s size, shape, and location, but they can’t always determine the exact type. Advanced techniques like MR spectroscopy analyze the chemical makeup of the tissue, helping distinguish tumor from other brain abnormalities. In some cases, a specialized PET scan using a tracer called 18F-FET can improve accuracy, particularly for identifying gliomas.
Imaging alone, however, can’t provide a definitive diagnosis. A tissue sample, obtained through biopsy or during surgical removal, remains the gold standard. A pathologist examines the cells under a microscope to determine the tumor type and grade, and increasingly, molecular testing identifies specific genetic mutations that influence both prognosis and treatment options. The WHO classification system, which is periodically updated by international panels of experts, provides the standardized framework for categorizing brain tumors based on these microscopic and molecular features.