Atypical meningiomas: WHO moved the goalposts?

Area of Science:

• Neurosurgery
• Pathology
• Oncology

Background:

• The World Health Organization (WHO) revised meningioma histological grading in 2000, standardizing definitions for atypical and malignant subtypes.
• Prior to 2000, the histological criteria for atypical and malignant meningiomas were less precisely defined, potentially leading to diagnostic variability.

Purpose of the Study:

• To retrospectively analyze meningioma cases operated on between 1993 and 2003 to assess the impact of the 2000 WHO classification changes on the proportion of tumors diagnosed as atypical.
• To investigate the epidemiology of meningioma, including associations with prior irradiation and surgical excision difficulty.

Main Methods:

• Retrospective analysis of 565 meningioma cases operated on between 1993 and 2003.
• Comparison of tumor grading proportions before and after the adoption of the 2000 WHO classification.
• Epidemiological analysis to identify risk factors and surgical outcomes associated with atypical meningiomas.

Main Results:

• A significant increase in the percentage of tumors graded as WHO II (atypical meningiomas) was observed after the 2000 WHO classification changes (from 18.3% to 23.1%, p=0.0408).
• Previous irradiation was found to be associated with atypical meningiomas (p=0.038).
• Surgeons reported greater difficulty in achieving complete excision of atypical tumors, with poorer intraoperative Simpson grades noted.

Conclusions:

• The 2000 WHO classification revision has led to a significant increase in the diagnosis of atypical meningiomas.
• The rise in atypical meningioma diagnoses may necessitate increased referrals for radiotherapy/radiosurgery, warranting examination of the evidence base for these treatments.
• Further investigation is needed to determine if these findings are replicated across other institutions and to evaluate the long-term implications for patient management.