Understanding Brain Cancer

How Is Brain Cancer Diagnosed?

Last updated and reviewed on June 16, 2026.

How Is Brain Cancer Diagnosed?

Last updated and reviewed on June 16, 2026.

Getting the right diagnosis for brain cancer is one of the most critical steps in the entire process. The accuracy of the diagnosis, not just brain tumor but the specific type, grade, and molecular features, directly determines which treatments are recommended and what outcomes to expect. The diagnostic process usually begins when a person develops symptoms that lead them to a doctor, or when an imaging scan done for another reason happens to reveal a brain mass. From there, a careful sequence of imaging, surgery, pathology, and molecular testing builds the complete picture.

What Tests Are Used to Diagnose Brain Cancer?

• Medical History and Neurological Examination: The process almost always starts with a thorough medical history and a neurological examination. The doctor asks about symptoms when they started, how they have changed over time, whether there is any family history of brain tumors or related genetic syndromes, and whether the person has any history of prior cancer or radiation therapy to the head. The neurological exam tests mental status, coordination, balance, strength, sensation, vision, hearing, and reflexes. This evaluation gives the first clues about where in the brain a problem might be.
• MRI of the Brain: MRI (magnetic resonance imaging) is the most important and most informative test for diagnosing brain tumors. It uses powerful magnets and radio waves to create detailed images of the brain without radiation. When a brain tumor is suspected, an MRI is done with and without a contrast agent (a dye injected into a vein that makes some brain tumors appear more clearly on the images because the dye leaks into the tumor where the blood-brain barrier is disrupted).

MRI can show the size and location of the tumor, whether it is solid or contains fluid-filled areas (cystic), and whether the tumor is disrupting surrounding brain tissue. Different MRI sequences provide different types of information: standard T1 and T2 images, fluid-attenuated inversion recovery (FLAIR) images (which are particularly helpful for showing swelling and infiltrating tumors), and post-contrast images all contribute different pieces to the picture.

More advanced MRI techniques are also used in brain tumor diagnosis at specialized centers:

• • MR Spectroscopy measures the chemical makeup of a tumor, which can help distinguish a tumor from other conditions that look similar on standard MRI, like radiation necrosis or abscess.
  • Perfusion MRI measures blood flow in the brain and can help assess how aggressive a tumor is.
  • Functional MRI (fMRI) maps which areas of the brain control critical functions like speech or hand movement. This is especially valuable before surgery to help surgeons understand what areas must be protected.
  • Diffusion Tensor Imaging (DTI) maps the white matter tracts (the "wiring" of the brain) so surgeons can plan an approach to the tumor that avoids damaging critical connections.

• CT Scan of the Brain: CT (computed tomography) scans are faster than MRI and are often used in emergency situations, for example, when a person arrives in the emergency room with a sudden, severe headache or new seizure. CT can quickly show hemorrhage (bleeding), obvious masses, and significant swelling. However, CT is much less detailed than MRI for evaluating brain tumors, and MRI is always preferred when time allows.

PET Scan: A PET scan uses a radioactive glucose (sugar) tracer that cancer cells take up more readily than normal cells. In brain tumor diagnosis, PET scanning can help distinguish between a living, active tumor and dead tissue (necrosis) that can sometimes look similar on MRI after radiation therapy. Specialized PET tracers that use amino acids (like FET-PET or FDOPA-PET) have become increasingly valuable for brain tumor imaging at specialized centers.

Biopsy and Surgery — The Only Way to Know for Certain: Imaging can strongly suggest the presence of a brain tumor, but the definitive diagnosis always requires looking at tumor tissue under a microscope. This means a biopsy is necessary.

Stereotactic biopsy: A minimally invasive procedure in which a needle is guided into the tumor using MRI or CT imaging as a map. The surgeon makes a small hole in the skull, inserts the needle, and removes a small core of tissue. This is used when the tumor is in a location that is too dangerous to remove surgically, or when surgery is not otherwise planned.

Open surgical resection with intraoperative biopsy: When surgery is planned to remove some or all of the tumor, the first tissue removed goes directly to a pathologist for immediate analysis (called a frozen section) to confirm it is tumor tissue before the surgeon continues.

Awake craniotomy: For tumors located in or near the speech, language, or motor areas of the brain, surgeons sometimes perform a craniotomy while the patient is awake. The patient can speak, follow commands, or move their limbs while the surgeon maps out exactly where those functions are, allowing maximum tumor removal with minimum risk to critical brain functions.

Pathology, Tumor Grade, and Molecular Testing: Once tissue is obtained, a neuropathologist examines it under a microscope. The pathologist determines the tumor type and assigns a grade (1 through 4) based on how abnormal the cells appear and how fast they are dividing. Higher grades indicate more aggressive tumors.

In recent years, molecular testing has become an essential part of brain tumor diagnosis. The 2021 WHO Classification of CNS Tumors now integrates molecular markers alongside traditional microscope findings to define tumor types. Key molecular tests include:

IDH mutation testing: IDH1 and IDH2 mutations are found in most low-grade and some high-grade gliomas and are associated with better outcomes compared to IDH-wild type tumors of the same grade.

MGMT promoter methylation testing: In glioblastoma, a methylated MGMT promoter predicts better response to temozolomide chemotherapy and longer survival.

1p/19q codeletion: When both chromosome arms 1p and 19q are deleted (lost), it defines oligodendroglioma and is associated with better treatment response and prognosis.

TERT promoter mutation, EGFR amplification, and others: are also tested in certain tumor types to refine diagnosis and guide treatment.

This integrated molecular diagnosis has fundamentally changed how brain tumors are classified and treated, and it is one of the reasons why expert neuropathology review at a specialized center matters so much.

Sources

1. 1. American Cancer Society. Tests for Brain and Spinal Cord Tumors in Adults. https://www.cancer.org/cancer/types/brain-spinal-cord-tumors-adults/detection-diagnosis-staging/how-diagnosed.html
   2. National Cancer Institute. Adult Central Nervous System Tumors Treatment (PDQ) – Patient Version. https://www.cancer.gov/types/brain/patient/adult-brain-treatment-pdq
   3. Louis DN, et al. The 2021 WHO Classification of Tumors of the Central Nervous System. Neuro-Oncology. 2021;23(8):1231–1251. https://pubmed.ncbi.nlm.nih.gov/34185076/
   4. Wen PY, et al. Glioblastoma in adults: a Society for Neuro-Oncology (SNO) and European Society of Neuro-Oncology (EANO) consensus review on current management and future directions. Neuro-Oncology. 2020. https://pmc.ncbi.nlm.nih.gov/articles/PMC7594557/ 
   5. National Brain Tumor Society. Diagnosing Brain Tumors. https://braintumor.org/brain-tumors/diagnosis/
   6. American Brain Tumor Association. Diagnosis. https://www.abta.org/about-brain-tumors/diagnosis/