Primary extradural meningioma, intraosseous type, WHO Grade I.
The imaging differential included fibrous dysplasia, metastases, and less likely meningioma. Typical meningiomas arise within the cranial vault in the subdural space, have a dural connection, and are thought to originate from neoplastic transformation of arachnoid cap cells. Meningiomas that arise outside the cranial vault without a dural connection are known as primary extradural meningiomas (PEMs). They comprise about 1-2% of all meningiomas and occur predominately in the head and neck region (e.g., calvarium, scalp, orbit, paranasal sinuses, nasopharynx, neck, and skin) with rarer reported sites including the adrenal gland, lung, mediastinum, paraspinal region, and limbs (5). Depending on their location, other descriptors for these neoplasms include intraosseous, calvarial, cutaneous, and ectopic. Intraosseous meningiomas, the subtype in this particular case, account for about 14% of cases of PEMs (1).
The Lang classification scheme(5) divides PEMs into Type I (extracalvarial with no attachment to bone), Type II (purely calvarial, located entirely within the bone of the skull), Type III (calvarial with extracalvarial extension). Type II and III lesions are further subclassified as skull base (B) or convexity tumors (C). According to this system, the present tumor is a Type IIB primary extradural meningioma. Based on data from the MD Anderson Cancer Center and a literature review, Lang et al. found that patients with skull base PEMs (Type IIB or Type IIIB) had significantly higher recurrence rates compared to those with convexity PEMs (Type IIC or IIIC): 26% compared with 0%.
Although the origin of intraosseous meningiomas is not definitively known, at least four theories of their pathogenesis have been put forth in the literature (3, 4). One explanation, the congenital/perinatal theory, is that rests of arachnoidal cap cells become entrapped within the cranial sutures either during birth or subsequent molding of the head. These arachnoidal rests are thought later in life to lead to the development of intraosseous meningiomas. A second explanation, the traumatic theory, posits that trauma causes entrapment of arachnoidal cells within fracture lines. A third explanation offered by Shuangshoti, et al.(6), posits that multipotential mesenchymal cells present within mesenchymal tissues of the body (including bone, cartilage, fibrous tissue, and adipose tissue) have the capacity to differentiate into meningothelial cells, which may then undergo neoplasia. A fourth explanation is that intraosseous meningiomas with an extradural epicenter may in fact arise from the dura. This hypothesis was put forth by Bassiouni et al.(2), who analyzed a series of 16 patients with presumed cranial vault PEMs. In their series MRI revealed dural enhancement at the tumor site in 11 patients and, intraoperatively, the tumor was judged by the surgeon to infiltrate the dura in 13 of the 16 patients. Their conclusion was that tumor infiltration of the dura should be assumed in cranial vault PEMs and that dural resection at the craniotomy site is recommended to minimize recurrence. They theorize that tumor cells may infiltrate the diploe through the haversian canals, thereby presenting as calvarial meningiomas. Although their study was restricted to cranial vault PEMs, it highlights the question of whether the majority of presumed primary extradural meningiomas may in fact arise from arachnoid cap cells that have migrated through bone by an unknown mechanism to produce an ectopic epicenter.
Contributed by Yan Qu,, James E Davis, John Chi, Santosh Kesari