Pathology of meningiomas.

Meningiomas are a diverse group of neoplasms that exhibit a wide range of morphologies and clinical behavior. They are generally accepted to originate from arachnoid cap cells within the leptomeninges. Classic histologic features include whorl formations, psammoma bodies, nuclear holes, and nuclear pseudoinclusions. Meningiomas are classified as benign, atypical, or anaplastic (grades I, II, or III) based on histologic features including mitotic activity, brain invasion, and presence of other minor criteria. There are numerous histologic variants of meningiomas, and some are associated with worse clinical outcomes and therefore are assigned a higher grade. The majority of meningiomas show diffuse positivity for vimentin and epithelial membrane antigen, supporting the dual mesenchymal and epithelial nature of meningothelial cells. The presence of an elevated proliferation index (as measured by Ki-67 immunohistochemical stain) and loss of progesterone receptor expression are associated with the higher grade. Pathologic features including histologic variants, grading criteria, and ancillary tests such as special and immunohistochemical stains are discussed.