Case 7 -

Meningiomas Timothy Smith and Cynthia Welsh

The term 'meningioma' was coined by Harvey Cushing as a tumor of cells of the meninges. In his treatise coauthored with Louise Eisenhardt there were twenty-seven types of meningiomas reflecting the diversity of cells normally present in the meninges. The current WHO classification includes only eleven subtypes, and three partially objective prognostic grades. They arise from arachnoid granulations where the CSF is resorbed, located along the dural sinuses. Meningiomas are well circumscribed slow growing tumors that expand slowly pushing the brain aside. Meningiomas on MRI usually enhance with contrast. They are slowly growing and approximately 20% recur after surgery within five years. Recurrences are believed due to incomplete removal because of the complex anatomy and because of recurrences in a field effect. The neurosurgeon finds a well circumscribed firm tumor that almost always is preoperatively identified as meningioma. Meningiomas arise from arachnoid but appear as dural based because the arachnoid is firmly attached to the dura. Embolization is often used as a technique to decrease the size of very large tumors, making them smaller more pliable and easier to remove. Meningiomas can assume many patterns in squash smears and cryostat sections, but the most common patterns are easily recognized. The cytology of meningioma on squash or smear preparations has plate-like cohesive cells arranged in sheets and variable sized groups without defined relationships to blood vessels. Large and small whorls are often seen in the smears. The background is clear and there are no inflammatory cells. Nuclei may show mild pleomorphism, often pseudoinclusions and small nucleoli. Mitoses are rare. Psammoma bodies are variably present. Cryostat sections of the most common pattern, the meningothelial pattern, show groups of plate-like, almost squamoid (the sometimes significant resemblance to squamous can be disastrous), cells having vague to marked whorling patterns intersected by fibrovascular septae. Elongate fascicles of fibroblastic cells with bundles of collagen form the next most common pattern named fibrous or fibroblastic. The third most common pattern, the mixed or transitional meningioma, is a blend of the meningothelial and fibroblastic patterns. Psammoma bodies are often present in the meningothelial and mixed patterns in the center of whorl formations. Fibroblastic meningiomas have Psammoma bodies infrequently. The remaining patterns are psammomatous (dominated by psammoma bodies), angiomatous (having many vessels), microcystic (having many microcysts and xanthoma cells), secretory (having many cells with distinct eosinophilic cytoplasmic inclusions), clear cell, chordoid (having foci resembling chordoma), lymphoplasmacytic-rich, and metaplastic (showing foci of bone, cartilage, or fat). None of the patterns have prognostic significance except probably the clear cell pattern. Papillary meningiomas have a poor prognosis and appear papillary because the atypical cells attached to vessels are discohesive resulting in the separated papillary structures. Rhabdoid meningiomas are really rare, have a rhabdoid phenotype and usually a poor prognosis. Other considerations: Meningiomas can superficially resemble metastatic carcinoma, but the usual low grade meningioma lacks the pleomorphism, apoptosis, mitoses and tumor diathesis. The squash prep or smear can also resemble a metastatic carcinoma, but the frozen section will provide clarification. Fibroblastic meningiomas can mimic schwannomas especially if the lesion is at the cerebellopontine angle. Meningiomas, schwannomas, and gliomas bulging from the brainstem all may occur at the cerebellopontine angle. At that site schwannomas usually have long sweeping, intersecting fascicles, biphasic patterns and twisted elongate nuclei. The biphasic pattern consists of the combination of the densely cellular Antoni-A pattern and the loosely cellular Antoni-B pattern. Thick walled vessels often with fibrinoid change are characteristic of schwannomas. Abnormal vessels may dominate over the spindle cell component so the erroneous diagnosis of vascular malformation could be rendered. Attachment to the eighth cranial nerve, expansion of the internal auditory canal and incipient Verocay bodies are found in schwannomas. Verocay bodies consist of parallel rows of nuclei separated by an eosinophilic anuclear zone. Admittedly some cases may need to be deferred to permanents. Poor staining quality and sectioning can allow a fibroblastic meningioma forming a bulge at the cerebellopontine angle to resemble an astrocytoma. Better staining and awareness of this phenomenon can aid in avoiding this trap. Higher grade, atypical and anaplastic, meningiomas occur but their identification is seldom necessary at the time of frozen section. Atypical meningiomas WHO grade 2 are identified by a subjective combination of the following features: increased mitoses (5mitoses/10 hpf), small sheeted cells in a patternless pattern, and necrotic foci. Malignant (anaplastic) meningiomas, WHO grade 3 are identified by their sarcomatous features, hemangiopericytic pattern, papillary pattern, tumor necrosis, brain invasion and increased mitotic rate. Embolized meningiomas will show necrotic foci with nearby atypia and mitoses that resemble features of malignancy. Knowledge of the embolization can help avoid this trap. The hemangiopericytoma of the meninges is malignant and can be recognized in cryostat sections, but that recognition is usually not necessary intraoperatively. Hemangiopericytomas of the meninges have a poor prognosis. Meningiomas that resemble soft tissue sarcomas are malignant. Genuine soft tissue sarcomas do occur in the meninges and virtually all types have been reported.