Teaching Case

A 73-year-old woman presented with a painless scalp swelling of the right frontoparietal region. The swelling was present for several months and had gradually increased in size. There was no history of previous trauma. Clinical examination did not reveal any neurological deficit.

CT (Fig. 1, a-c) showed a right-sided frontotemporal hyperdense intradiploic mass, with an anteroposterior diameter of 9 cm and a laterolateral diameter of 1,9 cm. The lesion was expansile and invaded the calvarium with segmental inhomogeneous sclerosis with thickening of the diploe and irregular contours of the lesion. The bony sclerosis extended anteromedially into the right posterolateral orbital wall. Additional MR imaging (Fig. 2) demonstrated a hypointense lesion both on T2-FLAIR and T1-weighted sequences. After gadolinium injection, no contrast enhancement was visualised. There was a small layer of fatty tissue (hypodense on CT – Fig. 1d, hyperintense on MRI – Fig. 2) on the inner side of the lesion, between the dura and the thickened bone, containing multiple vessels with flow voids.

Primary intraosseous – extradural meningiomas are rare lesions, invading the intradiploic space and the inner/outer table of the calvarium [1,2].  The majority of meningiomas are considered intradural, located in the subdural space [1–3]. Only 1-2% of all meningiomas are extradural and the primary intraosseous meningiomas represent a major subset of these extradural meningiomas [1].

A number of different theories [4–10] regarding the origin of intraosseous meningiomas have been postulated. In general, extradural meningiomas are derived from mesenchymal cells, more specifically from arachnoid cap cells [1,2,10].

Lang et al. [2] introduced a unifying nomenclature and classification scheme based on the position of the tumours to the cranium. Type I tumours are purely extracalvarial (without attachment to the bone), type II are purely calvarial (within the diploic mass), and type III have an extracranial extension with a soft-tissue component. Type II and III tumours are subdivided into skull base (B) or convexity (C) tumours.

The symptoms depend primarily on the location of the lesion [1]. As in this case, the majority of primary intraosseous meningiomas present as painless expansile masses without neurological deficits [11]. The tumours occur equally in both sexes.

Meningiomas present as osteoblastic (65%) or osteolytic (35%) [2,3] subtypes, although mixed versions have been reported. Unenhanced CT demonstrates a hyperdense skull lesion. Differential diagnosis with fibrous dysplasia is challenging. Fibrous dysplasia usually stops growing after puberty, whereas intraosseous meningiomas typically appear after puberty and continue to grow slowly [1]. The inner table is smooth in fibrous dysplasia, whereas intraosseous meningiomas often demonstrate irregularity of the inner table [1]. Use of MR imaging is reserved for better anatomic delineation of tumours that have an intradural or extracalvarial soft tissue component [3,4,7,11]. The tumours are typically hypointense on T1-weighted images and hyperintense on T2-weighted images [1,3,11]. After contrast administration, no enhancement is seen in the osteoblastic subtype, whereas the osteolytic subtype enhances homogeneously [4,7]. The dural tail sign, as often found with intradural meningiomas is not present in intraosseous meningiomas, however, gadolinium enhancement of the underlying dura may be noted [1].

An osteosarcoma could not be excluded, therefore a biopsy was performed. After histopathologic assessment (Fig. 3) the diagnosis was a primary intraosseous meningioma, WHO grade 1 (benign). Complete resection with reconstruction is the only potentially curative treatment [1–3,11], which was not performed in this case due to the absence of neurological deficit and pain, the age of the patient and the benign tumoral properties.

Written informed patient consent for publication has been obtained.