Characterization of intermediate type (WHO "atypical") meningiomas.

The intermediate type meningioma (formerly "meningioma variant with signs of increased proliferation activity"; WHO "atypical" meningioma) represents a meningioma group the mean Ki-67 index and the recurrence rate of which lie between those of the ordinary and the anaplastic type [Kolles et al. 1995]. In the study cited (n = 160) the percentage of recurrences was 9% in the common, 29% in the intermediate, and 50% in the anaplastic type. The present study focuses on 49 subsequently diagnosed meningiomas of the intermediate type. Apart from certain histopathological features, the most important independent factors associated with recurrence were Ki-67 indices covering the 95% confidence interval between 3.7% and 4.9%. At the light-microscopic level, however, meningiomas in general, and especially those supposed to belong to the intermediate type, are highly variable in tissue architecture and do, in contrast to the anaplastic type, not display frank histological features of anaplasia. Morphologically, the growth pattern of solid ("syncytial") sheets, micronecroses, and large distinct nucleoli are suspicious. Cytogenetically, independent of the loss of one chromosome 22, it has been shown that terminal loss of the short arm of one chromosome 1 (1p-) or complete loss of this chromosome were useful additional indicators of meningioma progression.