Distinct clinical outcome of microcystic meningioma as a WHO grade 1 meningioma subtype

To evaluate the clinicopathological characteristics, radiology, and long-term outcomes of microcystic meningiomas (MM) and compare it with other subtypes of meningiomas managed at a single neurosurgical center. A total of 87 consecutive patients who underwent surgical resection and were diagnosed as MM between 2005 and 2016 were enrolled for analysis. Clinicopathological, radiology, and prognostic information was collected and analyzed. Progression free survival (PFS) was compared with 659 patients with other subtypes of WHO grade 1 meningiomas and 167 patients with atypical meningiomas treated during the same period. Fifty six females and 31 males with MM were analyzed. Peri-tumor brain edema was frequent on T2 WI (85%).12 patients (13.8%) experienced tumor progression during the mean follow-up of 101.66 ± 40.92 months. The median PFS was unavailable, and the 5, 10, and 15 year progression-free rates were 96.9%, 84.0%, and 73.9%, respectively. Univariate COX analysis demonstrated skull base location and higher Ki-67 index as significant negative prognostic factors for PFS (P < 0.05); multivariate analysis identified tumor location and Ki-67 index as independent factors (P < 0.01), as well. Of note, the PFS of MM was worse than other WHO grade 1 subtypes (P < 0.001), but better than atypical meningiomas (P < 0.001), and the PFS differences were retained even when the analysis was limited to the patients receiving GTR (P < 0.05). The PFS of MM was worse than other WHO grade 1 subtypes and better than atypical meningiomas. Skull base location and higher Ki-67 index were independent negative prognostic factors in MM.


Introduction
Meningiomas are the most common primary tumors in the central nervous system and account for approximately 39% of all intracranial neoplasms [1]. According to the newest WHO grading system in 2021, meningiomas can be classified into three grades and fifteen histological subtypes [2]. Approximately 80% of meningiomas are WHO grade 1, which are usually benign and do not exhibit aggressive behaviors. Microcystic meningioma (MM), which was first named by Kleinman et al. in 1980, was included in the WHO grading standard and assigned to WHO grade 1 in 1993 [3]. MM is a rare subtype that accounts for about 1.6% of the total meningiomas [4]. Like other WHO grade 1 meningiomas, MM was reported to show benign behaviors both clinically and histologically. MM often occurs in the supratentorial area. Common clinical manifestations are headache, seizure, and contralateral weakness due to the tumor compression on brain tissue [5][6][7]. Histologically, MM shows vacuolation and microcapsule-like structure under the background of mucus; it is composed of star-shaped or fusiform cells arranged in a vortex shape, with the loose intercellular structure and the vacuolar cytoplasm [8]. Some MM patients display radiological characteristics that are not typical for low-grade meningiomas, which can be difficult to distinguish from other intracranial neoplasms like atypical meningioma or glioma [9]. Recently, a study showed that chromosome 5 polysomy represented the molecular genetic characteristics of MM [10], suggesting that MM could be a tumor type biologically distinct from the other grade I meningiomas.
Up to now, very few studies comprehensively studied the clinicopathological, radiological, and prognostic characteristics of this relatively rare subtype of meningioma. In this study, we collected 87 consecutive patients with MM treated at a single neurosurgical center to evaluate the clinicopathological characteristics, radiology, and long-term outcomes of MM. We also compared the outcomes of MM with other grade 1 histological subtypes and atypical meningioma.

Radiological data
Preoperative magnetic resonance images (MRI) were analyzed in 40 patients (46%) since those were not available for 47 patients (54.0%) due to upgrading of the MRI imaging system. The features of MRI were evaluated by two experienced radiologists, both blinded to clinical and histopathological information. The radiological data including T1WI, T2WI, FLAIR, contrast enhanced T1WI were collected. The following features were investigated and analyzed: tumor size, cystic formation, dural tail sign, peritumoral brain edema (PTBE), heterogeneous enhancement, marginal and reticular enhancement. Tumor size was computed as the measurement of maximal diameter based on MRI. Edema index (EI) was calculated by the ratio of edema area to tumor area on the maximum diameter plane. PTBE was evaluated with T2WI sequence and divided into four levels, namely absent (EI < 0.01), mild (0.01 < EI < 0.50), moderate (0.51 < EI < 1.99), and severe (EI ≥ 2.0).

Immunohistochemical review
All tumor specimens were fixed in 10% formalin, embedded in paraffin, and cut into 4 mm thick sections, and subjected to Hematoxylin and Eosin staining and immunohistochemistry. These were reviewed and re-confirmed by two board-

Patient follow up
Patients were followed up through phone or out-patient service after surgery according to the meningioma followup protocol [11]. The last follow-up was on October 30, 2021. Postoperative complications, progression-free survival (PFS) and postoperative treatment were recorded. Recurrence was confirmed with the enhanced T1WI MR images. PFS was defined as the time between surgery and tumor progression. Thirty patients were lost during the follow-up, and the remaining 87 patients were included in the final analysis. 659 patients with other histological subtype grade 1 meningiomas and 167 patients with atypical meningioma operated during the same time period whose follow-up data were available were enrolled for PFS comparisons.

Statistical analysis
Statistical analysis was performed using R software (Version 3.4.1). The R packages used in our study included "survival, survminer, ggplot2, Proc, Hmisc, foreign, and tidyverse". Student's t-test and Mann-Whitney U test compared continuous and categorical variables, respectively. Kaplan-Meier method and the long-rank test were used to evaluate PFS. Univariate and multivariate Cox regression analysis was used to identify independent predictors of MM progression. A two-sided P-value < 0.05 was defined as statistically significant.

Histopathology results
Histopathological review by two neuro-pathologists confirmed that all specimens were MM. EMA was negative in two patients (

Analysis of prognostic factors
All clinical characteristics, including age, gender, tumor location, extent of tumor resection, Ki-67, PR, symptom duration and preoperative KPS score, were analyzed as potential prognostic factors via univariate and multivariate Cox regression analyses. In the Pearson Correlation Test, all correlation coefficients were below 0.6, indicating that these variables were independent (Supplementary Fig. 2). Univariate Cox analysis demonstrated that lower Ki-67 index (Fig. 1a), and non-skull base location (Fig. 1b) were factors significantly associated with longer PFS (   (Fig. 2a). PFS of MM was significantly poorer than the other WHO grade 1 subtypes combined (P = 0.0006) (Fig. 2b) and better than atypical meningioma (P < 0.001) (Fig. 2c). In contrast, there was no significant difference between the PFS of 4 common  (Fig. 2e). The PFS of MM was worse than the other grade 1 subtypes (P = 0.0005) (Fig. 2f) and better than atypical meningioma (P < 0.001) (Fig. 2g). And again, patients with the 4 non-MM subtypes receiving GTR showed no difference in PFS (P = 0.982) (Fig. 2h). We further compared Ki-67 index and the extent of resection among these subtypes. The Ki-67 index of atypical meningioma was significantly higher than MM and other grade 1 subtypes (P < 0.001) (Fig. 3a, b), while there was no difference between MM and other grade 1 histological subtypes (P = 0.203) (Fig. 3c). Adjusted Chi-square test showed that the STR rate of atypical meningiomas was higher than WHO grade 1 meningioma (P = 0.002), while no difference (P = 0.205) was observed between MM and other WHO grade 1 subtypes (Fig. 3d).

Discussion
MM is a relatively rare subtype of WHO grade 1 meningioma [4]. The clinical, radiological, and genetic characteristics of MM are not clear due to the limited number of patients. Studies concerning MM are either case reports or small case series [3,5,8,[12][13][14]. Our study comprehensively analyzed the clinical, radiological, and prognostic characteristics of 87 MM patients managed at our neurosurgical center. To the best of our knowledge, this is the largest MM series from a single academic institution. In this series from 2005 to 2016, the incidence of MM was 0.86%, which is similar to previous reports [6]. MM is usually characterized by the onset of slow progressive symptoms due to the compression of adjacent structures [15]. In our study, the most common symptoms were headache, contralateral weakness, and seizure. The mean age of our patients was 51.8 years, which is similar to other benign meningiomas.
The female/male ratio of our patients was 1.81, which is in accordance with meningiomas in general but higher than previous series [3,6,7]. The most common locations of MM were convexity and parasagittal/falx, which is also in accordance with previous studies [3,6,16]. GTR could be achieved in most patients. Among the small proportion of the patients with STR (8.1%, 7/87), only one patient with Simpson grade IV resection received gamma knife radiotherapy. Long-term follow-up through radiological examinations was recommended for these patients with STR [17,18]. Radiological analysis showed that severe PTBE was common in MM [6,19,20]. In our series, as many as 34 cases (85%) had varying degrees of PTBE. The reticular enhancement was considered as the unique radiological presentation of MM in several studies [20][21][22], but its sensitivity was not high [3,6]. Only three patients presented with this imaging characteristic in our cohort ( Supplementary Fig. 3a-h), while heterogenous and marginal enhancement was common. Lin et al. divided MM into three types according to their imaging and clinical features in their series of 69 patients [6]. These three types of MM were significantly different in sex ratio, severe PTBE incidence and extent of tumor resection. In our series, correlative analysis between radiological presentation and clinical features was limited since radiological data were unavailable in over half (47/87) of the patients.
Genetic background of MM is still not clear. Few studies reported the molecular genetic analysis of MM. Common genetic alterations including NF2, TRAF7, KLF4, AKT1, SMO and POLR2A mutations were frequent in other grade 1 meningiomas but not in MM [23][24][25]. Ketter et al. recently detected 16 cases of hyper-diploid meningioma in 667 consecutive patients, of which 6 cases were MM, indicating a potential hyper-diploid feature of MM [26]. In our series, molecular data was unavailable. Molecular characteristics of MM warrant further investigations.
An interesting discovery in our series was that MM has a worse PFS outcome than the other WHO grade 1 meningiomas, with as many as 12 patients (13.8%) experiencing tumor progression. The 5, 10 and 15 year PFS rate of MM was all lower than that of non-MM grade 1 diseases. Two possible factors (Ki-67 index and extent of tumor resection) that could lead to the progression survival difference were evaluated in our own series. No Ki-67 index difference was observed between MM and the other grade 1 tumors. Even limited in patients with GTR, MM still had a significantly worse outcome than other grade 1 subtypes, suggesting a distinct post-surgery natural history of MM. We reviewed published studies concerning the outcome of grade 1 meningiomas to evaluate outcomes between MM and other grade 1 subtypes. Prognostic information of MM was only available in two studies that reported a recurrent rate of 12-1.6%, respectively. However, Lin et al. reported the 1.6% recurrent rate from a short median follow-up time of 49 months. Together with our current work, MM tended to have a worse outcome than other subtypes. Six studies focusing on other subtypes of grade 1 meningiomas  reported the recurrent rate ranging from 1.54 to 13.3%, all these lower than 13.8% in our MM series (Table 2), supporting a distinct post-operative clinical course of MM [3,6,[27][28][29][30][31][32]. Therefore, longer-term and close follow-up should be recommended for MM patients [18]. Our work warrants further investigations to understand why MM had a worse outcome than other subtypes. In the studies reported by Lin et al. and Kalani et al., extent of resection was significantly associated with the outcome of MM [3,6]. Our survival analysis demonstrated that other than extent of resection, tumor location and Ki-67 index were also associated with PFS. Multivariate analysis identified skull-base location and higher Ki-67 index as independent risk factors for shorter PFS. Therefore, MM patients with skull-base tumor and/or high Ki-67 index should have regular, close, and long-term follow-up after operation.

Limitations
This study has two limitations. Firstly, the radiological data was only available in 40 patients, making comprehensive analysis of the radiological characteristics difficult. Secondly, our study is a retrospective study, and selection bias and recall bias may exist.

Conclusion
Skull base location and higher Ki-67 index were independent negative prognostic factors in MM. Importantly, the PFS of MM was worse than other WHO grade 1 subtypes and better than atypical meningiomas.

Author contributions
All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by LR, LH and ZB. The first draft of the manuscript was written by LR and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
Funding This work was supported by grants from the National Natural Science Foundation of China (82072788 to YG), and the Shanghai Sailing Program (20YF1403900 to LYH).

Data availability
The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request.

Conflict of interest
The authors have no relevant financial or non-financial interests to disclose.
Ethical approval This clinical study was approved by the Human Subjects Institutional Review Board at Huashan Hospital, Fudan University.

Informed consent
The consent process was omitted due to the retrospective nature of our study.

Consent to publication
The authors affirm that human research participants provided informed consent for publication of the images in Supplementary Fig. 3a-h.