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

(Low-grade)

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

Grade II

(Low-grade)

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

Grade III

(High-grade)

  •  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

(High-grade)

  •  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.
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