**6. Benign meningiomas or WHO grade I meningiomas**

In most of cases, WHO grade I meningiomas are diagnosed. They present a benign clinical course, with rare mitoses and occasional pleomorphic nuclei. Various architectural patterns have been described, whose most frequent are meningothelial, fibroblastic, and transitional meningiomas. Meningothelial or syncytial meningiomas are characterized by a high cellular density with cells disposed in packed sheets. Fibroblastic meningiomas are similar to schwan‐ nomas, with elongated cells. Cellular whorls and psammoma bodies (mineralized whorls) are most common in the transitional type, which present intermediate features between the meningothelial and the fibroblastic subtype. Other rare variants are the psammomatous meningiomas, characterized by multiple psammoma bodies, the angiomatous meningiomas, which are extremely vascularized with scarce foci of meningothelial cells and the secretory meningiomas, with lumina formed by cytokeratin-immunoreactive cells and containing PAS positive material.

Benign meningiomas are considered cured, if a complete resection is performed. Thus, a simple surveillance is indicated in cases where a gross total resection is possible (Simpson grade 1 or 2) with serial cerebral MRI, without any complementary treatment. A very small portion of benign meningiomas progresses to more aggressive variants (<2%) [38], but when consider‐ ing only recurrent tumors, the prevalence becomes significantly higher (14%) [39]. The clinical behavior and the malignant potential of meningioma are still matter of debate. Many au‐ thors applied the model of clonal progression to meningiomas with a progressive appear‐ ance of more aggressive cellular subpopulations according with the presence of genetic imbalances and molecular modifications [40, 41].

Meningiomas show immunohistochemical positivity for epithelial membrane antigen (EMA) in 80% of cases. They also express other epithelial markers such as vimentin and cytokeratin. Antileu 7 and glial fibrillary acidic protein (GFAP) stains, characteristics for schwannomas and gliomas, respectively, are uniformly negative. Immunohistochemistry may thus help in the differential diagnosis in difficult cases.

An elevated labeling for Ki-67 may denote a more aggressive lesion [42, 43].
