Key points

Incidence

A majority of meningiomas are considered primary intradural meningiomas (PIMs) and are located in the subdural space. By contrast, extradural meningiomas arise in locations other than the dura, such as the skin, nasopharynx, and neck. Extradural meningiomas comprise 1% to 2% of all meningiomas. Primary intraosseous meningioma is a term used to describe a subset of extradural meningiomas that arise in the skull. They represent approximately two-thirds of all extradural meningiomas. A review of the literature reveals approximately 200 reported cases of intraosseous meningioma. This number has increased dramatically over the past decade. An article in 1995 presented the 36th case to the literature, whereas a publication 5 years later reviewed 168 cases of calvarial meningioma. Rather than a higher incidence, this increase in published cases likely represents a combination of improved histopathologic capabilities, such as immunostaining, as well as increased penetrance of radiographic techniques, such as MRI.

This article reviews the radiographic and clinical findings of patients with primary intraosseous meningiomas. Differential diagnosis, nomenclature, and treatment options are also discussed. The literature regarding these tumors is reviewed.

Incidence

A majority of meningiomas are considered primary intradural meningiomas (PIMs) and are located in the subdural space. By contrast, extradural meningiomas arise in locations other than the dura, such as the skin, nasopharynx, and neck. Extradural meningiomas comprise 1% to 2% of all meningiomas. Primary intraosseous meningioma is a term used to describe a subset of extradural meningiomas that arise in the skull. They represent approximately two-thirds of all extradural meningiomas. A review of the literature reveals approximately 200 reported cases of intraosseous meningioma. This number has increased dramatically over the past decade. An article in 1995 presented the 36th case to the literature, whereas a publication 5 years later reviewed 168 cases of calvarial meningioma. Rather than a higher incidence, this increase in published cases likely represents a combination of improved histopathologic capabilities, such as immunostaining, as well as increased penetrance of radiographic techniques, such as MRI.

This article reviews the radiographic and clinical findings of patients with primary intraosseous meningiomas. Differential diagnosis, nomenclature, and treatment options are also discussed. The literature regarding these tumors is reviewed.

Etiology

Different theories exist regarding the origin of intraosseous meningiomas. Three main theories exist. First, arachnoidal cells on blood vessels or nerves traversing the skull may explain the origin of some intraosseous meningiomas. Cellular dedifferentiation within the skull could also possibly explain the formation of an intraosseous meningioma. Some investigators observed a possible trend for intraosseous meningiomas found predominantly near previous fracture sites or at suture lines, most commonly the coronal or pterion sutures. Intraosseous meningiomas are thought to arise from arachnoidal cap cells caught in-between cranial sutures during birth, with subsequent modeling of the infant skull leading to intraosseous meningomas. The bony orbit and frontoparietal skull are the most common locations; however, they can theoretically arise from any location where there is cranial suture closure. Extradural tumors may arise from cells that become misplaced after differentiation into meningocytes or arachnoid cap cells. Intraosseous meningiomas are thought to originate from arachnoid cap cells trapped in the cranial sutures, which may occur during molding of the cranium at birth. Another theory involves meningothelial cells becoming trapped within skull fractures or sutures as a result of trauma. Recently published case series, however, do not usually support these 2 theories. A recent report published in 2000 reviewed 168 cases of calvarial meningiomas reported in the literature. Only 14 of these cases (8%) were associated with a cranial suture. A separate report reviewed 36 cases in the literature and found that only 5 (14%) were associated with a history of head trauma in the region of the tumor. Lastly, the concept of ectopic arachnoid cap cells giving rise to intraosseous meningiomas should be considered. In general, the meninges are derived from mesenchymal cells. Thus, extradural meningiomas could arise in numerous unusual locations as a result of aberrant differentiation and/or misplacement of multipotent mesenchymal stem cells.

Clinical presentation

Similar to many cranial and intracranial lesions, the clinical presentation and resulting differential diagnosis depend primarily on the location of the lesion. For example, calvarial intraosseous meningiomas most commonly present as slowly growing scalp masses, with possible relationship to a cranial suture. These are typically firm and painless, with normal overlying skin, and may be detected incidentally. Neurologic signs and symptoms are usually absent. In contrast, skull base intraosseous meningiomas may present with cranial nerve deficits, such as ophthalmoplegia or visual field problems, or signs and symptoms related to mass effect, such as proptosis. Symptoms, such as hearing loss, tinnitus, headache, and vague sensations in the head, are also reported. One primary difference in clinical presentations between intradural and extradural intraosseous meningiomas is the potential for scalp involvement.

Extradural meningiomas, including intraosseous meningiomas, are reported to occur with the same frequency in each gender, unlike intradural meningiomas, which occur twice as frequently in women as in men. Like intradural meningiomas, they predominantly occur later in life, but extradural meningiomas also have a second peak incidence during the second decade of life. The tumors are nearly uniformly solitary, although 1 case report presented a patient with 2 separate lesions. In this patient, an initial diagnosis of fibrous dysplasia was made after CT showed hyperostosis of the temporal and sphenoid bones. Two years later, a repeat CT scan showed the interval development of hyperostosis in the occipital bone and biopsy diagnosed intraosseous meningioma.

Radiographic appearance

Like their dural based counterparts, intraosseous meningiomas may induce hyperostosis. In these cases, conventional radiographs demonstrate hyperdensity associated with the lesion, although superimposed bony structures may limit the usefulness of this radiographic modality. Skull radiographs can detect abnormalities in 30% to 60% of cases of intraosseous meningioma, including hyperostosis, thinning of bone, irregular foci of calcification, and atypical vascular markings. Similarly, CT with bone windows shows a focally thickened, hyperdense, intradiploic lesion expanding the calvaria and destroying the cortical layers of the skull. The tumor is usually hyperdense on unenhanced CT, ranging from 65 to 85 Hounsfield units, and enhances densely after contrast administration similar to intradural meningiomas. The expansion of bone and ground-glass appearance of this type of intraosseous meningioma may appear radiographically similar to fibrous dysplasia. Differential diagnosis includes hyperostosis, Paget disease, fibrous sclerosis, sclerotic metastasis, and inflammatory lesions of bone.

Alternatively, primary intraosseous meningiomas may present as an osteolytic skull lesion. It is thought that a majority of these meningiomas are osteoblastic (65%), with 35% osteolytic. Similar expansion of the calvaria and cortical destruction is seen, although the intradiploic lesion is hypodense on plane films or CT. A report in 1995 reviewed the radiographic findings of the 34 published cases of intraosseous meningioma. Radiographic evidence of hyperostosis was noted in 20 (59%) cases, whereas 11 (35%) showed osteolytic changes in the surrounding bone. The remaining 2 cases (6%) revealed a mixed picture of both osteolysis and hyperostosis. As of 2007, only 16 cases of the rare osteolytic subtype of intraosseous meningioma had been described in the literature, which represents less than 10% of reported cases overall.

MRI findings are similar to those for intradural lesions and allow better delineation of tumors that have extracalvarial soft tissue extension. The tumors are typically hypointense on T1-weighted images and hyperintense on T2-weighted images. Prominent, homogeneous enhancement after gadolinium administration is typical. The tumors do not exhibit the dural tail often found with intradural meningiomas, but gadolinium enhancement of the underlying dura may be noted. This dural enhancement could be secondary to dural irritation or tumor invasion.

A recent report evaluated angiographic findings of 10 patients with intraosseous meningioma. Consistent findings in a majority of patients included an enlarged, tortuous feeding artery, a dense tumor blush, and early venous drainage. Branches of the external carotid artery fed the tumor in most patients.

Bone scintigraphy using technetium Tc 99m diphosphonate has been used to assess the response of an intraosseous meningioma to adjuvant therapy in a patient with an unresectable lesion. Bone scintigraphy has also been used to demonstrate a tripling in size of an intraosseous meningioma over a 5-year period.