Primary intraosseous meningioma of the calvaria

Clinical History

An 63-year-old woman presented with a scalp swelling of the left frontoparietal region. It was present for the last 6 years and had gradually increased in size. The swelling was not painful, and there was no history of trauma. The patient had no neurologic deficit.

Imaging Findings

The CT study after intravenous contrast injection was performed and showed:
In bone window (Fig. 1, 2):
A left-sided frontoparietal intradiploic mass expanding the calvaria with segmental homogeneous thickening and sclerosis of diploe, inner and outer table. The inner table was irregular (Fig. 1).
In soft tissue window (Fig. 3, 4):
Cerebral parenchyma and meningeal spaces were normal.

Discussion

The meningiomas arising in locations outside the dural compartment are called ectopic, extradural, calvarial, cutaneous, extracranial, extraneuraxial or intraosseous meningiomas [1].
Extradural meningiomas constitute 1 to 2% of all meningiomas [2]. Primary intraosseous meningiomas are a subtype of primary extradural meningiomas that arise in bone [3] and represent approximately two thirds of all extradural meningiomas [4]. The vast majority of primary intraosseous meningiomas involve the calvaria [5].
Primary extradural meningiomas are classified as purely extracalvarial (type I), purely calvarial (typeII), or calvarial with extracalvarial extension (type III). According to the site of location of the tumour, Lang et al [4] further subdivided type II and III lesions into convexity (C) or skull base (B) forms. Because the cerebral parenchyma and meningeal spaces are normal, the case presented here is a type IIC meningioma.
Primary intraosseous meningiomas occur with approximately the same frequency in each sex in the fifth decade [4].
Clinically, as in our case, the majority of the primary intraosseous calvarial meningioma present as painless expansile masses with normal neurological findings [6]. These lesions may be asymptomatic and detected incidentally [7].
The radiographic appearance of intraosseous meningiomas depends largely on their location. They are typically either the osteoblastic or osteolytic subtype, although mixed versions have been reported. The majority of intraosseous meningiomas are osteoblastic subtype [8, 9]. CT with bone windows shows a focally thickened, hyperdense intradiploic lesion expanding the calvaria. The tumour is usually hyperdense on unenhanced CT and there is no enhancement in osteoblastic subtype after contrast administration. More rarely, primary intraosseous meningioma may present as an osteolytic skull lesion, typically cause thinning, expansion, and interruption of the inner and outer cortical layers of the skull. The lesions are similarly hyperdense on a nonenhanced CT and enhance homogeneously after contrast administration [8, 10]. MR imaging provides a better anatomic delineation in the evaluation of the soft tissue component and extradural extension of the lesion. But, MRI is less sensitive than CT for the detection of bone lesions. On MRI, the tumours are typically hypointense on T1 weighted images and hyperintense on T2-weighted images. Prominent homogeneous enhancement after Gadolinium administration is typical. Enhancement of the underlying dura may be noted. This dural enhancement could be secondary to dural irritation or tumour invasion [11].
Our patient underwent an elective biopsy procedure of the parietal bone by the neurosurgery service. The histopathology results indicated an intraosseous meningioma. There was no nuclear pleomorphism, necrosis, or high mitotic activity indicative of malignancy. It had a grade I according to WHO grading.
Complete resection with reconstruction is the only potentially curative treatment. Recurrence rates for all locations are estimated to be between 10% to 23% [12, 13].

Differential Diagnosis List

Final Diagnosis

Primary intraosseous meningioma of the calvaria.

URL:	https://www.eurorad.org/case/13477
DOI:	10.1594/EURORAD/CASE.13477
ISSN:	1563-4086