Breaking down brain tumour terminology

21 May 2026 | Blog, Campaigning and research, Collaborations, News

When someone is diagnosed with a brain tumour, they may be overwhelmed by the need to understand a new language. This article aims to provide a shortcut to that new language so that patients, caregivers, colleagues, employers, journalists, policymakers and others can better understand what is being discussed and why the words matter.

Understanding the language used to describe brain tumours

Brain Tumour Support, The Brain Tumour Charity, Brain Tumour Research and brainstrust – the brain cancer people have collaborated on this blog post to provide accurate and consistent information for the whole brain tumour community.

Brain tumours are as unique as the person diagnosed

Low-grade, high-grade, benign, malignant, brain tumour, brain cancer – talking about brain tumours is rarely straightforward. Even though they mean different things, these terms are often used interchangeably or side by side, which can be confusing and contradictory. What do they actually mean?

Brain tumours are incredibly complex. There are more than 100 types, each with its own patterns of growth, behaviour and response to treatment. Even tumours with the same name can behave differently from one person to another.

Malignant versus non-malignant

Malignant and non-malignant are terms used throughout medicine to describe how a tumour behaves, including its potential to grow, invade surrounding tissues, and spread to other parts of the body. These terms provide doctors with information about how serious it may be and what kind of care is likely to be needed. Broadly speaking, malignant means cancer and non-malignant means not cancer.

Being told that something isn’t malignant usually feels like good news. But for brain tumour patients, even a non-malignant brain tumour can be life-changing, or sometimes life-limiting.

Malignant brain tumours are often referred to as brain cancer. They grow quickly, invade healthy brain tissue and are more likely to return after treatment. They are often diffuse. This means that they have no clear boundary, making them difficult to remove. They can spread to other areas within the brain and spinal cord. Malignant brain tumours have a worse prognosis than non-malignant tumours.

Non-malignant brain tumours usually grow slowly and tend not to spread, and may also be referred to as ‘benign’ by clinicians and online resources. They have clearer borders, which means that surgeons may find them easier to remove, and once removed, they are less likely to return. However, this depends on where the tumour is and which important brain structures are nearby. Therefore, ‘benign’ can be misleading as it can make it sound as though the tumour is harmless.

Why benign is not fine

Each brain tumour patient’s experience is unique. For some, describing a tumour as “non-malignant” or “benign” can be reassuring, but for others, this does not represent their experience.

Someone may be told that their non-malignant brain tumour is inoperable, for instance, because it’s located in a vital part of the brain that would be damaged during surgery, or because treatment may have serious side effects.

And the skull is a fixed, enclosed space, so any tumour, malignant or not, has no room to expand and can press on and damage nearby brain tissue. This can lead to serious symptoms, affect quality of life, and, in some cases, be life-threatening. In other words, any brain tumour has the potential to cause harm.

Let’s talk about the word “benign”, the medical antonym of “malignant” and consigned to the “not dangerous” category. I have poor balance, muscle weakness, fatigue, memory loss, reduced function in my right eye and ear, slurred speech, and take strong medication that induces high dependency. I fall over most days. I walk into things. I’ve been refused entry into establishments on account of drunkenness (I’m teetotal), which unfortunately, the above symptoms can present as. I rely on my “good” (left) side and have just about mastered the art of carrying two drinks in one hand. Medically, “benign” means “not harmful”. Anecdotally, “benign” has little application in the world beyond hospital. Benign is not fine…an excerpt from Thomas’s story. Read in full on the brainstrust website.

While some tumours remain non-malignant throughout a person’s life, others may be non-malignant at diagnosis but may progress over time into malignant tumours. This process is called malignant transformation.

The behaviour and impact of a tumour diagnosis can also vary from person to person. In short, each person’s diagnosis is unique to them. We cannot make assumptions based on a one-word description.

High-grade and low-grade – a different way to classify brain tumours

Scientists and clinicians working with brain tumours use tumour grades to better reflect how tumours are likely to grow and spread, and how they should be treated. These grades are based on guidance from the World Health Organisation (WHO), which classifies brain tumours using features seen under the microscope and, increasingly, genetic and molecular information.

A pathologist – a doctor who diagnoses disease by studying tissues and cells – determines the grade by examining a tumour sample taken during surgery or a biopsy. They study the cells under a microscope to investigate their structure and growth, and use specialist equipment to identify certain genetic and molecular markers. Generally, tumours with cells that resemble healthy brain cells are given a lower grade, while those with more abnormal-looking cells are assigned a higher grade.

• Grade 1 tumours are usually slow-growing and less likely to spread
• Grade 2 tumours are usually slow-growing but have the potential to transform to a higher grade
• Grade 3 tumours are fast-growing and can spread to other parts of the brain and the spinal cord
• Grade 4 tumours are the fastest-growing. They often return after treatment and can spread to other parts of the brain and sometimes the spinal cord

In broad terms, grades 1 and 2 are often called low grade, while grades 3 and 4 are called high grade.

Some tumours contain a mixture of cells with different grades. The tumour is graded according to the highest grade of cell it contains, even if the majority of cells are low grade. This is because even a small area of a more aggressive tumour can influence treatment decisions.

Why aren’t brain tumours staged like other cancers?

Stage and grade are not the same thing: grade describes how the tumour looks and behaves biologically, while stage describes how far a cancer has spread in the body. For many types of cancer, staging helps clinicians predict outcomes and guide treatment decisions. However, primary brain tumours – tumours that start in the brain rather than spreading there from somewhere else – rarely spread beyond the brain or spinal cord. The extent of their growth and spread is not enough to predict how they will behave or respond to treatment. Instead, the grading system is used, which captures a wider range of factors to better predict tumour behaviour and potential impact.

There’s no one-size-fits-all

Each brain tumour is unique. This is why patients should talk to their medical team before making decisions about treatment, clinical trials, additional medicines or supplements. A person’s tumour type, molecular markers, general health and other medical conditions, individual values and practical circumstances can all affect which options are available.

Related articles and information:

• Brain tumour data – Statistics that shock but can also bring hope
• Brain tumour types
• Low and uncertain grade brain tumours