There are more than 120 types of brain tumours. Today, most medical institutions use the World Health Organization (WHO) classification system to identify brain tumours.
The WHO system classifies brain tumours by cell origin and how the cells behave, from the least aggressive (benign) to the most aggressive (malignant).
Some tumour types are assigned a grade, ranging from Grade I (least malignant) to Grade IV (most malignant), which signifies the rate of growth. There are variations in grading systems, depending on the tumour type.
The classification and grade of an individual tumour help predict its likely behavior.