Answer: Microcystic meningioma

Histology: The lesion consists of haphazardly arranged cells with variable clear, vacuolated and eosinophilic cytoplasm with microcystic architecture. The cells are admixed with abundant blood vessels, some of which have thin walls and some of which are hyalinized. The cells have enlarged nuclei with evenly dispersed chromatin and punctate nucleoli. No mitotic figures or necrosis is identified. Scattered psammomatous calcifications are present. A special stain for PAS (periodic acid-Schiff) is negative.

Discussion: This lesion represents a variant of WHO grade I meningioma, a microcystic meningioma. The presence of microcysts and psammoma bodies, with a negative PAS stain, help make the diagnosis. The meningiomas that are by definition WHO grade II are the “2 C’s”: clear cell and chordoid. The meningiomas that are by definition WHO grade III are “PR”: papillary and rhabdoid (remember that meningiomas label positively for the progesterone receptor, or “PR”). The remaining histologic subtypes, which include meningothelial, fibrous, psammomatous, angiomatous, microcystic, and secretory, are all WHO grade I. An increased mitotic rate will increase the grade (4-19/10HFP is atypical or grade II, >20/10HPF is anaplastic or grade III), as will the presence of three or more of the following features: hypercellularity, the presence of small cells, prominent nucleoli, tumoral necrosis, and sheet-like growth. The most common genetic abnormalities in meningiomas are mutations in NF2 (neurofibromatosis type 2 gene, located on chromosome 2) and loss of chromosome 22q.

Clear cell meningiomas are characterized by clear cells with PAS positive cytoplasm (glycogen) and no microcysts; in addition, secretory meningiomas are also PAS positive. Hemangioblastomas have neoplastic cells with clear cytoplasm (lipid) that are positive for inhibin associated with numerous capillaries. Both hemangioblastomas and renal cell carcinomas can be seen in patients with von Hippel Lindau (VHL) syndrome, or can occur sporadically with VHL mutations. Clear cell renal cell carcinomas also have cells with clear cytoplasm and delicate vessels, and they label with Pax8, RCC, and CD10. Myxopapillary ependymomas contain small papillary structures with myxohyalin cores, lined by cuboidal cells which are positive for GFAP and make mucin (PAS positive).

Reference(s):
– Mawrin C1, Perry A. Pathological classification and molecular genetics of meningiomas. J Neurooncol. 2010 Sep;99(3):379-91.
– Riemenschneider MJ1, Perry A, Reifenberger G.Histological classification and molecular genetics of meningiomas. Lancet Neurol. 2006 Dec;5(12):1045-54. Erratum: Reimenschneider MJ, Perry A, Reifenberger G. Histological classification and molecular genetics of meningiomas. Lancet Neurol 2006:5:1045–54. In the last line of panel 1 on page 1045, the sentence “Meningiomas of any type or grade with high proliferation index and/or brain invasion” has been mistakenly listed under the heading of WHO grade III lesions. Meningiomas with high proliferation index and brain invasion are associated with a greater likelihood of recurrence and/or aggressive behaviour. However, this does not inevitably imply a classification as anaplastic tumour of WHO grade III.