Meningiomas have a wide range of histopathologic appearances. These include:
- WHO grade I: meningothelial, fibrous (fibroblastic), transitional (mixed), psammomatous, angiomatous, microcystic, secretory, lymphoplasmacyte-rich, and metaplastic.
- WHO grade II: atypical, chordoid, and clear cell.
- WHO grade III: anaplastic (malignant), rhabdoid, and papillary.
Malignant behavior, including brain invasion, may occur with any grade of meningioma.
These types of tumors are known to be induced by ionizing radiation, with an average time interval to tumor appearance of 19 to 35 years, depending on the dose of radiation. Most patients with radiation-induced meningiomas have a history of low-dose radiation to the scalp for tinea capitis; the second largest number of radiation-induced meningiomas occurs in patients who have received high-dose radiation for primary brain tumors. Multiple meningiomas often occur in patients with neurofibromatosis 2 (NF2) and in other, non-NF2 families with a hereditary predisposition to meningioma.
The most common cytogenetic alteration in meningiomas involves a deletion of chromosome 22. Molecular genetics findings indicate that approximately 50% of meningiomas have allelic losses that involve band q12 on chromosome 22. Allelic losses of chromosomal arms 6q, 9p, 10q, and 14q are seen in both atypical and anaplastic meningiomas. Genetic and cytogenetic alterations accumulate with progression from WHO grade I to WHO grade III lesions. Mutations in the NF2 gene have been detected in as much as 60% of sporadic meningiomas. After surgical resection, benign meningiomas (WHO grade I) recur in about 7% to 20% of cases, atypical meningiomas (WHO grade II) recur in 29% to 40% of cases, and anaplastic meningiomas recur in about 50% to 78% of cases. Malignant histologic features correlate with shorter survival times; one series has reported a median survival of less than 2 years for patients with anaplastic meningiomas. Brain invasion indicates a greater likelihood of recurrence, regardless of histology.
Hemangiopericytoma of the CNS was long considered a meningioma, but it is now recognized as a mesenchymal, nonmeningothelial tumor histologically indistinguishable from hemangiopericytomas occurring in soft tissue and with a tendency to recur and to metastasize outside the CNS. It is a highly cellular and richly vascularized tumor that is almost always attached to the dura. Histologic criteria for grading are not firmly established; however, these types of tumors appear to correspond histologically to WHO grade II or III. Meningeal hemangiopericytomas comprise approximately 0.4% of all primary CNS tumors. These types of tumors tend to appear at a younger age than meningiomas, and they occur more often in men than in women. No specific chromosomal abnormalities or molecular genetics exist with this tumor. After surgical resection, most hemangiopericytomas recur; in two series, they recurred in 91% and 85% of cases after 15 years. Postoperative radiation therapy delays recurrence. Most meningeal hemangiopericytomas eventually metastasize. In a series of 28 patients who survived primary resection, the probability of tumor-related death was 61% at 15 years.