WHO Grading System

The World Health Organization (WHO) classifies brain tumours by cell origin and how the cells behave, from least to most aggressive. Some tumour types are assigned a grade to signify their rate of growth and to help predict behaviour.

Many non-malignant brain tumours are classified as Grade I or II and malignant tumours as Grade III or IV; however, mixed-grade tumours are possible.

The distinction between non-malignant and malignant tumours can be challenging. Some non-malignant (or low-grade) tumours can be as serious as those classified as malignant (high-grade) if they are in an inaccessible location, such as the brainstem. Conversely, some malignant tumours can be successfully treated. 

WHO Tumour Grading System

Grade I


  • Slow-growing cells
  • Cells appear almost normal under microscope
  • Least malignant / aggressive
  • Usually associated with long-term survival 

Grade II


  •  Relatively slow-growing cells
  • Slightly abnormal cell appearance under microscope
  • Can invade nearby healthy tissue
  • Can recur as a higher grade tumour

Grade III


  •  Actively reproducing abnormal cells
  • Cells appear abnormally under microscope
  • Affects nearby healthy tissue
  • Tumour tends to recur, often becoming a higher grade tumour

Grade IV


  •  Abnormal cells that reproduce rapidly
  • Very abnormal cell appearance under microscope
  • Form new blood vessels to maintain rapid growth
  • Areas of dead cells in centre (necrosis)
Update: In 2016, the World Health Organization (WHO) made changes to it's classification of brain tumours. Read this Ask The Expert article which explains some of the changes and how this affects the diagnosis and management of brain tumours.
Share This

Featured Story

Courtney’s Story of Stability

Stability. It’s a strange concept when you have what it known to be a progressive, life long illness. You hear the words, “Your tumour growth is stable” and for a moment you think someone is playing the world’s worst prank on you.

Learn more


Stephen's Story: "I have faith that we will meet again"

Stephen and I chatted on what should have been his 32nd Wedding Anniversary. Stephen and Susan were married for 30 years and were best...

Learn more

Tommy's Story: Fellowship recipient

Dr. Tommy Alain, the very first research Fellow funded by Brain Tumour Foundation of Canada through the William Donald Nash Brain...

Learn more

Upcoming Events

  • 17/Jan/2018: Windsor Support Group: Meets at the United Way, Unit A1, 300 Giles Blvd. East... Learn more >
  • 17/Jan/2018: Calgary Support Group: Meets at Wellspring Calgary, 1404 Home Road NW, Calgary, Alberta, T3B 1G7... Learn more >
  • 17/Jan/2018: Niagara Region Support Group: Meets at Wellspring Niagara, 3250 Schmon Parkway, Thorold, ON, L2V 4Y6... Learn more >
  • 17/Jan/2018: Winnipeg Support Group: Sturgeon Creek United Church, 207 Thompson Drive, Winnipeg, MB... Learn more >
View All Events >
Thank you to the donors whose contributions make this website and all programs, services and research possible.

Copyright © 2018 Brain Tumour Foundation of Canada. Charitable Registration #BN118816339RR0001
35 Years