World Health Organization (WHO) grade I benign meningioma includes nine subtypes : meningothelial, fibrous, transitional, psammomatous, angiomatous, microcystic, secretory, lymphoplasmacyte-rich, and metaplastic meningioma [1,5,6,8,9,19,21]. Among these, metaplastic meningioma is an extremely rare subtype, defined as containing focal or widespread mesenchymal components including osseous, cartilaginous, lipomatous, and myxoid or xanthomatous tissue, singly or in combination [1,2]. In addition, metaplastic meningiomas are often referred to as lipomatous, osseous or xanthomatous, based on their dominant mesenchymal components [19].

The clinical and radiological characteristics of meningiomas, the most common extraaxial primary brain tumor, have been widely described. The unique radiological features associated with their histology have also been delineated. Microcystic meningioma, characterized by the formation of intercellular microcysts, show the distinguishing radiological feature of markedly high signal intensity with severe peritumoral edema in T2-weighted images [4,13,16]. Sclerosing meningioma also show peculiar radiological features, such as very low signal intensity on T2-weighted images due to their abundant calcification [10]. Secretory meningioma are characterized by an intracellular lumen containing an eosinophilic secretion (i.e., a pseudopsammoma body), which shows a characteristic high intensity in up to 96% of cases. They may also exhibit relatively high apparent diffusion coefficient values compared with typical meningiomas [11,13].

Due to their rarity, the clinical and radiological features of metaplastic meningiomas have been poorly described. Thus, the study goal was to systematically review all available surgically confirmed metaplastic meningiomas at our center, to determine whether they have unique clinical and/or radiological characteristics.

This study was performed in accordance with the ethical standards of the International Research Board of Seoul National University (IRB No. H-2005-163-1125). We retrospectively analyzed patients whose diagnosis with meningioma was surgically confirmed at our institute between 2004 and 2018, during which time 1814 total cases were identified. Eleven consecutive cases were found to have been pathologically confirmed as metaplastic meningioma. These patients’ clinical, radiological, histopathological, and surgical records were obtained from electronic databases and reviewed in detail. These patients (four males and seven females) had a mean age of 60 years.

For radiological analyses, preoperative magnetic resonance (MR) and computed tomography (CT) images were collected, including: attenuation (high, iso-dense, low), calcification, and fat component (absent versus present) for CT images; T1- and T2-weighted signal intensity (high, iso, or low), enhancement pattern (homogeneous versus heterogeneous), and peritumoral edema (absent versus present) were collected for MR images. Histopathological reviews were repeated by a neuropathologist.

From 2004 to 2018, a total of 1814 surgically confirmed meningioma cases were seen at our institute. Metaplastic meningioma was diagnosed in 11 cases, representing 0.8% of WHO grade I meningioma and 0.6% of all meningiomas during the 15-year study period. The details of these meningioma grades and subtypes are shown in Fig. 1.

Although meningioma are the most common primary extra-axial tumors, some subtypes are unfamiliar to many neurosurgeons. This is primarily due to their low incidence, especially since WHO grade I meningioma include nine subtypes, based on their histopathological features. Among these, metaplastic meningioma is among the rarest. In 2018, Zouaoui and colleagues [22] reported a nationwide descriptive epidemiology of meningioma. In their 15-year study in France, the incidence of metaplastic meningioma was 0.04% (62 of 13038 newly diagnosed and histologically confirmed meningioma). Another epidemiologic study also showed the extreme rarity of metaplastic meningioma, with an incidence of 0.21% (15 of 7084 cases) [18]. Similarly, the proportion of metaplastic meningioma was 0.8% in our 15-year surgical experience with meningiomas. Thus, our data reconfirm that it is among the rarest meningioma subtypes.

Another notable study finding was our initial evaluation of meningioma distribution according to WHO grade. During our 15-year study period, WHO grade I meningioma accounted for 1376 (75.9%) cases : 354 (19.5%) and 84 (4.6%) cases were WHO grades II and III, respectively. The two epidemiologic studies described above reported 89.2% and 91.9% WHO grade I, 9.3% and 5.2% WHO grade II, and 1.5% and 2.9% WHO grade III meningioma, respectively [20,22]. Thus, our incidence rates of WHO grades II and III meningiomas appear slightly higher than those reported previously. However, we must acknowledge several inherent study limitations. The study was retrospective and conducted at a single-center over 15 years. Possible selection bias was included due to the rarity and heterogeneity of metaplastic meningioma and the protocol did not reflect changes over time in the WHO classification criteria.

Metaplastic meningioma is referred to by several different names based on the presence of metaplastic changes to the dominant mesenchymal component : osseous, lipomatous, cartilaginous, myxoid and xanthomatous meninigoma [21]. The term ‘metaplastic’ is used because the transformed neoplastic cells demonstrate the full histological characteristics of the cells they mimic [19]. Lipomatous metaplastic meningioma are made up of adipocyte resembling fatty cells with a signet-ring appearance [3,6,21]. Myxoid metaplastic meningioma are characterized by their excessive hyaluronic acid and chondroitin sulfate [5,7,9,12]. Osseous subtypes are confirmed by the presence of mature bone tissue and tumor cells in regions containing no psammoma bodies [1,14,15,17,18]. A recent literature review by Choi et al. [2] described 26 metaplastic meningiomas, including 12 osseous subtypes, nine lipomatous subtypes, four xanthomatous subtypes, and one cartilaginous subtype. In our study, we were able to identify subtypes within the 11 metaplastic meningiomas, including nine lipomatous, four osseous, and two xanthomatous. No cartilaginous metaplastic meningioma was identified. Most metaplastic meningioma in our study were lipomatous or osseous metaplastic meningioma, similar to previous studies.

On radiological examination, the lipomatous type metaplastic meningioma demonstrated peculiar radiological features associated with a fatty component (Table 1, case 1 of Fig. 2). Although calcification is commonly observed in a meningioma, osseous type metaplastic meningioma had a distinct calcification on CT and MR images (Table 1, case 9 of Fig. 2). No other unique clinical characteristics of metaplastic meningioma were found.

The primary study purpose was to determine whether metaplastic meningiomas have unique clinical or radiological characteristics. We were unable to identify any unusual characteristics that might help distinguish metaplastic meningioma from other meningioma subtypes. However, toward further defining metaplastic meningioma, we note some common radiological and histological characteristics, such as the representative cases of lipomatous and osseous metaplastic meningioma. Despite the study limitations described above, we expect this study to confer a better understanding of the nature of these rare tumors.

We investigated a rare metaplastic meningioma subtype based on our 15-year surgical experience with meningiomas. Metaplastic meningiomas are classified based on their mesenchymal components, some of which have unique radiological characteristics. Further investigation will be necessary to describe the nature of these rare tumors more clearly.