Answer: Meningioma, WHO grade I

Histology: The lesion is comprised of bland spindled cells arranged in clusters and whorls with focal intranuclear pseudoinclusions and frequent psammoma bodies. No mitotic figures are identified. No nuclear pleomorphism or necrosis is present. No brain invasion is identified.

Discussion: Meningiomas are common brain tumor arising from the dura covering the brain, and they are the most common benign brain tumor. They are more common in women and are usually solitary, but may be multiple, particularly when associated with neurofibromatosis type 2 (NF2), which is a defect in the NF2 gene encoding merlin on chromosome 22. Up to 60% of isolated or sporadic meningiomas also display somatic mutations in the NF2 gene (1). By immunohistochemistry, meningiomas are positive for progesterone receptor (PR) and EMA, which can be used in difficult cases to support a meningothelial origin.

The majority of meningiomas (90%) are WHO grade I. Certain histologic variants are higher grade by definition. Clear-cell, chordoid and atypical meningiomas are WHO grade II. Papillary, rhabdoid and anaplastic meningiomas are WHO grade III (malignant). The remaining histologic patterns include meningiothelial, psamomatous, secretory, fibroblastic, angiomatous, and microcystic and others. Atypical meningiomas may be any histologic pattern, but also include either an increased mitotic rate (4-20 mitoses/10 HPF), or three of the five following features: hypercellularity, prominent small cell compmonent, prominent nucleoli, sheet-like growth, and geographic necrosis. Anaplastic meningiomas have >20 mitoses/10 HPF or frank anaplastic cytology. The presence of brain invasion is not a criteria used for grading of meningiomas by the WHO, but its presence should always be reported, as it is associated both with incomplete resection as well as a higher risk of recurrence.

Reference(s):
– Whittle IR, Smith C, Navoo P, Collie D. Meningiomas. Lancet. 2004;363(9420):1535-43.