The current WHO 2007 classification divides meningiomas into a 3-grade prognostic hierarchy. Recent literature evokes two pathways to disease progression in meningiomas akin to a comparable paradigm in gliomas, but without similar prognostic connotation: de novo anaplastic meningioma (better prognosis), and transformed meningioma (worse prognosis). We present two adult cases of transformed meningiomas that display a spectrum of morphologic progression. Case 1 at presentation showed a random admixture of meningothelial, atypical and anaplastic meningioma. The tumor recurred as anaplastic meningioma. Case 2 presented as a chordoid meningioma, but recurred as anaplastic meningioma mainly at the invasive front in transition with residual chordoid pattern. Of interest, portions of tumor also showed papillary configuration. In accordance with the dire prognosis for anaplastic meningioma, both patients succumbed to their disease within 2 months of recurrence. The present study highlights two main points: First, that proper recognition of focal high-grade areas in a heterogeneous low-grade meningioma (case 1) provides critical morphologic clues to spatial histologic progression and predicts aggressive biologic behavior, as evidenced by progression to frankly anaplastic meningioma at recurrence. Second, the presence of papillary in addition to anaplastic areas, in the recurrence of a previously diagnosed chordoid meningioma supports the ostensibly heightened transforming potential of grade II meningiomas, but also reflects on the morphologic heterogeneity of high-grade meningiomas, and their potentially diverse pathways of progression. We propose that grading of meningiomas as outlined by WHO is of more critical prognostic import than histologic sub-typing, and must include a thorough survey of the tumor-brain interface. Future molecular genetic correlates, akin to those characterized in gliomas, could help stratify prognostic subcategories to refine meningioma grading, and govern optimal therapeutic strategies.
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