Intraosseous meningiomas are a subtype of extradural meningiomas. They originate within bone, and have a different presentation and radiographic appearance than classic extraaxial meningiomas. These tumors have also been referred to as primary intraosseous meningioma.
There is some historical variability regarding the nomenclature of intraosseous meningioma. Ectopic, secondary, extra-calvarial, cutaneous, extracranial, primary extra -neuraxial, and extradural have all been applied to describe meningiomas that grow in places other than the subdural compartment. This entry focuses on meningiomas that arise in the calvarium, and are called intraosseous meningiomas because they arise from bone.
For primary extradural meningiomas, the following classification has been suggested:
- Type I, purely extracalvarial tumors
- Type II, purley calvarial tumors
- Type III, calvarial tumors with extracalvarial extension
The classification is further divided into convexity (C) or skull base (B).
Since meningiomas arise from mesenchymal cells, theoretically, they may arise from misplaced multipotent mesenchymal stem cells. Alternatively, they may arise from arachnoid cap cells that have also arisen aberrantly. Another hypothesis is that arachnoid cap cells become trapped in cranial sutures. None of these theories explains all tumors that have been seen.
Intraosseous meningiomas are rare. Extradural meningiomas make up 1-2% of all meningiomas. There have been fewer than 150 case reports of intraosseious meningoma in the literature (as of 2007). Unlike other meningiomas which have an increased incidence in women, intraosseous meningiomas are probably evenly distributed between genders.
Convexity intraosseous meningiomas usually grow as painless scalp masses, with a possible relationship to a cranial suture. Skull base meningiomas usually grow as painless masses, and may present with cranial nerve deficits.
Their appearance is depedent upon whether or not they are osteoblastic or osteolytic. Osteoblastic tumors tend to induce hyperostosis. This results in hypderdensity on conventional radiographs and CT scan. There may be expansion of bone and a "ground glass" appearance. Rarely, these tumors may be ostolytic and induce a lytic lesion with thinning and expansion of the skull on plain radiographs or CT. On MRI, intraosseous meningiomas are typically hypointense on T1 weighted images, and hyperintense on T2 weighted images. Homogeneous enhancement is typical, but the intraosseous component may not enhance, only the adjacent dura.
Since these tumors may appear osteoblastic or osteolytic, a variety of lesions may be considered. For osteoblastic lesions, one should consider osteoma, osteosarcoma, Paget's disease, or fibrous dysplasia. For osteolytic types, one should consider hemangioma, chondroma, chondrosarcoma, dermoid, epidermoid tumor, brown tumor, multiple myeloma, plasmacytoma, giant cell tumor, aneurysmal bone cyst, eosinophilic granuloma, or metastatsic cancer.
These specimens often require decalcification which can take several days. Upon histological examination, these tumors frequently exhibit classic features of meningioma including psammoma bodies, whorls, and eosinophilic tumor cells. The meningotheliomatous meningioma subtype is most common.
The treatment of choice is surgical excision, particularly for convexity tumors. For skull base tumors, treatment may be more difficult as complete excision may not be possible.
- Edler, J.B., Atkinson, R, Zee, Chi-Shing, Chen T.C. "Primary Intraosseous Meningioma". Neurosurg Focus 23 (4)::E13, 2007
- Agrawal, V, Ludwing, N, Agrawal A, Bulsara, K.R., "Intraosseous Intracranial Meningioma". AJNR, Feb 2007