"The use of the word "atypical" to denote a higher grade, refers to the histological appearances (see below). This is perhaps unfortunate as it can cause confusion among clinicians who are used to refer to something being 'atypical' when it doesn't look like the 'usual' garden-variety tumour/condition. One should therefore not confuse "atypical meningioma" with histological variants, many of which have unusual imaging and histological features but are nonetheless WHO grade 1 tumours (see meningioma article for further discussion of histological variants)."
"Generally, it is impossible to distinguish between benign (WHO grade 1) from atypical (WHO grade 2) and anaplastic (WHO grade 3) meningiomas with any confidence based on general morphology. The most reliable feature is the presence of lower apparent diffusion coefficient values (reflecting higher cellularity) 3,4."