Intracranial meningiomas of atypical (WHO grade II) histology.

Atypical (WHO grade II) meningiomas occupy an intermediate risk group between benign (WHO grade I) and anaplastic (WHO grade III) meningiomas. Although grade II meningiomas have traditionally been recognized in only about 5% of cases, after changes in diagnostic criteria with the current 2007 WHO standards, they now comprise approximately 20-35% of all meningiomas. Given the magnitude of this change, much work is now needed to solidify the adoption of these standards, to render inter-observer and inter-institutional comparisons more uniform, and to more carefully define the incidence of grade II histology. However, it is clear that they carry a several-fold increased risk of recurrence, as well as an increased rate of mortality. We will discuss the definition, diagnosis, and treatment of patients with atypical meningioma; review the current phase II cooperative trials; and draw attention to some questions timely for pre-clinical and clinical research.