Characteristic features and proposed classification in 69 cases of intracranial microcystic meningiomas

Microcystic meningioma (MM) is a rare subtype of intracranial meningiomas, with clinical and radiologic features not well characterized in the literature. Based on our experience, we propose a classification system of intracranial MMs. We reviewed the medical records, radiographic studies, and operative notes of a group of consecutive patients with intracranial MM. The mean age of the 69 patients was 46.8 ± 10.6 years (range, 21–75 years). Three types of intracranial MMs could be identified. Type 1 MMs presented as a solid lesion, hypointense or isointense on T1WI, hyperintense on T2WI, and homogeneous or heterogeneous enhancement, and were found in 43 patients (67.2%). Type 2 MMs represented signals similar to CSF both on T1WI and T2WI, and faint reticular enhancement with marginal enhancement, and these were found in 7 patients (10.9%). Type 3 MMs consisted of cystic-solid or cystic lesion and were found in 14 patients (21.9%). Significant differences were observed among the different types of MMs for the following variables: sex, presence of severe peritumoral brain edema (PTBE), and extent of tumor resection. Females were found in all of patients with type 2 MMs, but were only 35.7% of those with type 3 MMs (P = 0.018). Severe PTBEs were more common among patients with type 1 MMs (55.8%) than among those with type 2 (14.3%) and type 3 MMs (14.3%) (P = 0.007). Type 1 MMs (97.7%) were associated with a significantly higher rate of gross total resection compared with the other two types (71.4 and 78.6%) (P = 0.019). Total length of hospital stay after craniotomy ranged from 4 to 30 days (median, 8 days). There were no significant differences in progression-free survival among the three types of MMs (P = 0.788). The current classification identifies three distinct types of intracranial MM based on their radiological findings and growth patterns. The type 1 MMs are more commonly associated with severe PTBE. Type 2 and Type 3 MMs have a higher predilection towards parasaggital location with venous involvement and therefore have a lower rate of gross total resection.