Malignant Transformation of Meningiomas.

Meningioma is the most common intracranial tumor. Sometimes, meningiomas can develop malignant transformation (MT). In this review, we review the incidence of MT of meningiomas. The incidence of MT of grade 2 meningiomas is likely to be higher than benign meningiomas. Approximately 1% to 4% of WHO Grade 1 meningiomas may undergo MT, while about 26% to 33% of Grade 2 meningiomas experience MT. Time to MT of grade 2 meningiomas seemed to be shorter than MT of grade 1 meningiomas. The time for Grade I meningiomas to undergo MT is approximately 5 years, while Grade II meningiomas typically experience MT in about 3 years. Several risk factors may be associated with MT, including non-skull base location, high mitotic Index, a larger primary tumor size, shorter recurrence time interval and male. Potential molecular mechanisms of MT include chromosomal abnormalities (Chromosome 22q deletion, NF2 gene mutation, loss of chromosome 1p), genomic alterations (FOXM1, CDKN2A/B and TERTp), and meningioma cancer stem cells. Secondary meningiomas may have poor tumor control rates and overall survival rates than primary meningiomas. Besides, the role of radiotherapy in MT of meningiomas is unclear. Major concerns are whether radiotherapy can induce MT of meningiomas, and whether radiotherapy can prolong time to MT through long term control of meningiomas. This review summarizes the MT of meningiomas, and may provide the direction for further study of meningiomas.