The accuracy of meningioma grading: a 10-year retrospective audit.

Although descriptive classifications of meningioma subtypes are well established, there has been inconsistency in the categorization of meningiomas into benign, atypical and anaplastic groups. The aim of this study was to reassess the incidence of atypical (grade II) meningiomas over a 10-year period by applying the World Health Organization (WHO) 2000 classification system. A secondary aim was to determine if grade II and III tumours were becoming more common. Sections of 314 meningiomas resected between 1994 and 2003 were retrieved from the archives of the Western General Hospital's neuropathology unit in Edinburgh. They were reassessed and graded by using the WHO 2000 classification system. The reviewers were blind to the original classification and grading. There was a gradual increase in the numbers of meningiomas being resected annually over the 10-year period. On reclassification, 78% of the meningiomas were classified as grade I, 20.4% as grade II and 1.6% as grade III. With regard to grade II meningiomas classified by using the WHO 2000 classification system, 38.1% had originally been classified as grade I prior to 2000, whereas 13.6% had originally been classified as grade I after 2000. In most cases, reclassification was due to formal counts of mitotic figures. Atypical meningiomas are diagnosed more frequently under the current WHO classification system than they were under the previous classification systems. Although the current WHO (2000) classification is more prescriptive than its predecessors, interobserver variability is likely to remain because of the subjective nature of some of the criteria.