Atypical meningiomas: WHO moved the goalposts?

The histological grading of meningiomas underwent substantial revision and standardization in a WHO review of 2000. Prior to this the histological definition of atypical and malignant meningiomas was less tightly defined. We conducted a retrospective analysis of all meningiomas operated on between 1993 and 2003 in our unit (n=565), to assess whether the WHO changes had altered the proportion of tumours diagnosed as atypical. The percentage of tumours graded WHO II (atypical meningiomas) has increased significantly since these changes were adopted (18.3-23.1%, p=0.0408). We also examined the epidemiology of meningioma, finding that previous irradiation is associated with atypical meningiomas (p=0.038) and surgeons find that complete excision is also more difficult with atypical tumours (p=0.010), reporting poorer Simpson grades intraoperatively before the tumour grade is known. The increase in atypical lesions caused by the grading changes may lead to a corresponding increase in the numbers of patients referred for radiotherapy/radiosurgery, and we examine the evidence base for this strategy and whether our experience is replicated in other units.