Intraosseous meningioma

Findings

Figure 1 and Figure 2: Bone windows from axial CT images reveal expansion and sclerosis of the left sphenoid bone.
Figure 3: Axial CT section on brain windows demonstrates displacement of the optic nerve medially with bony expansion and an associated soft tissue component.
Figure 6, Figure 7 and Figure 8: T2, Axial FLAIR and gradient echo demonstrate expansion and diffuse low signal in the left sphenoid.
Figure 9 and Figure 10: Axial and coronal T1-weighted contrast enhanced images demonstrate minimal left sphenoid enhancement (Figure 7) and a conspicuous avidly enhancing, dural-based left temporal mass consistent with a meningioma.

Differential diagnosis:
- Intraosseous meningiomas
- Paget disease
- Fibrous dysplasia
- Sclerotic metastases
- Inflammatory lesions of the skull base


Diagnosis: Intraosseous meningioma


Meningiomas without a dural component are considered ectopic and can occur in a variety of locations including the subcutaneous tissues, paranasal sinuses, orbit, neck, salivary glands, the perineural sheath of cranial nerves and the calvaria. Purely intraosseous meningiomas (without an associated dural component) can be mistaken for osseous pathology including fibrous dysplasia, Paget disease and sclerotic metastases. Inflammatory lesions of the skull base, such as aspergillosis, may also give a similar appearance and should be considered in the proper clinical setting (sinus disease, diabetes, immunocompromise).

Meningiomas arise from arachnoid meningothelial “cap” cells. The trapping of “cap” cells in the cranial sutures during birth and subsequent modeling of the infant skull is thought to produce intraosseous meningiomas. This explains their predilection for the cranial sutures with the bony orbit and frontoparietal skull most commonly involved. Other proposed etiologies of intraosseous meningiomas include antecedent trauma, resulting in dural trapping within fracture lines and the multipotent nature of mesenchymal cells which give rise to the meninges and a variety of tissue types (including bone) which have been identified within meningioma pathologic specimens.

CT findings of meningiomas include a hyperdense (75%) smooth mass abutting the dura. Hyperostosis is a common occurrence in meningiomas; this may be secondary to local calvarial invasion, reaction to the adjacent dural mass or a primary osseous lesion. Calcification is identified in 25% of meningiomas which may cause focal areas of low signal intensity on MR imaging. The T1 weighted MR imaging appearance of meningiomas is an isodense extraaxial mass with a dural base and gray matter buckling. The appearance of meningiomas on T2 weighted images is isointense to cortex with a rim of CSF or trapped CSF clefts. Occasionally meningiomas may appear high in signal on T2 weighted imaging, indicating syncytial or angioblastic histology. Visualized flow voids may also be present within larger meningiomas, implying their vascularity. More than 95% of meningiomas demonstrate avid, uniform enhancement on post-contrast images. Meningiomas may display varying degrees of associated parenchymal edema.

Like their dural counterpart, purely intraosseous meningiomas demonstrate a 2:1 female predominance (in the 36 reported cases) and are generally considered benign. Intraosseous meningiomas demonstrate diffuse sclerosis and bony expansion on CT images. The MR appearance of intraosseous meningiomas is diffuse low signal of the expanded bone, the presence of an associated enhancing dural component greatly aids in the diagnosis by excluding both primary bone pathology and metastases. In meningiomas with both dural and intraosseous components, it is impossible to determine the site of origin as the osseous involvement may be the result of secondary invasion. Intraosseous meningiomas are treated with surgical excision when symptomatic. Cranial reconstruction with bone grafting is often necessary. Any remaining intraosseous component, secondary to involvement of sutures should be followed with serial radiographic examinations with adjuvant radiation for recurrent symptoms or lesion progression.