CHARACTERIZATION OF INTERMEDIATE TYPE (WHO ATYPICAL) MENINGIOMAS

The intermediate type meningioma (formerly ''meningioma variant with s i,ans of increased proliferation activity''; WHO ''atypical'' meningio ma) represents a meningioma group the mean Ki-67 index and the recurre nce rate of which lie between those of the ordinary and the anaplastic type [Kolles et al. 1995]. In the study cited (n = 160) the percentag e 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 hi stopathological features, the most important independent factors assoc iated with recurrence were Ki-67 indices covering the 95% confidence i nterval between 3.7% and 4.9%. At the light-microscopic level, however , meningiomas in general, and especially those supposed to belong to t he intermediate type, are highly variable in tissue architecture and d o, 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 s uspicious. Cytogenetically, independent of the loss of one chromosome 22, it has been shown that terminal loss of the short arm of one chrom osome 1 (1p-) or complete loss of this chromosome were useful addition al indicators of meningioma progression.