Primary Extradural Meningioma

Background:

• Primary extradural meningioma (PEM) is considered as a tumor with its main mass centered extracranially and may or may not have some form of invasion toward the outer layer of the dura.
• The classification for PEMs is as follows: purely extracalvarial (Type 1), purely calvarial (Type 2), or calvarial with extracalvarial extension (Type 3). The type 2 and type 3 lesions can be further subdivided into convexity (C) or skull base (B) forms.
• The reported incidence of PEMs is 1–2% of all meningiomas, with up to 68% of PEMs involving the calvarium.
• The most commonly involved locations for PEMs are the frontoparietal and orbital regions. Other sites of involvement include the paranasal sinuses, parapharyngeal space, along the perineural sheath of cranial nerves, and sites even more distal from the cranium such as the lung, adrenal glands, or paraspinal tissues.
• PEMs are more prone to develop atypical or malignant changes compared with intracranial meningiomas.
• PEMs with an atypical appearance are categorized in a WHO grade II classification and suggest more aggressive histologic features such as increased mitosis rate, hypercellularity, or tumor invasion into adjacent structures. One study has found a recurrence rate and death rate of 11.1% for atypical PEMs. Incompletely resected WHO grade II tumors are considered for postoperative radiotherapy or chemotherapy.

Clinical Presentation:

• Depends on the location and associated mass effect
• Typically presents as a palpable or visible scalp swelling. Other presentations may include proptosis or cranial nerve/spinal cord compression.

Key Diagnostic Features:

• Multiplanar CT with additional bone windows and brain MRI with contrast-enhanced sequences will aid in delineation of the tumor, assessment for cortical destruction, and also assessment for the intraosseous and extraosseous extension of the tumor.
• CT typically demonstrates diffuse sclerosis and expansion of the bone. PEMs have similar MRI signal characteristics to intracranial meningiomas: T1 isointense, T2 isointense/hyperintense, uniform avid contrast enhancement, and avid uptake on SPECT scan.
• Atypical appearances of a PEM may include cystic, fatty, or necrotic appearance, nonhomogeneous CT or MRI density, heterogeneous/absent contrast enhancement, heterogeneous radiotracer uptake, absent dural tail, or lytic changes of the adjacent bone.
• In our case of an atypical PEM with a lucent and lytic focus, lack of homogeneous contrast enhancement, and minimal radiotracer uptake, the diagnosis was not evident via imaging. As such, histopathologic assessment was undertaken to confirm the diagnosis while also excluding differential diagnoses.

Differential Diagnoses:

• Metastasis: May have a similar radiologic appearance though is likely to have avid contrast enhancement and radiotracer uptake.
• Langerhans cell histiocytosis: typically a punched-out lytic lesion with a beveled edge due to asymmetric involvement of the inner and outer tables.
• Gorham disease (vanishing bone disease): Results in progressive osteolysis along with proliferation of thin-walled vascular channels, and may spread to involve adjacent bone or soft tissue structures.
• Generalized lymphatic anomaly: A systemic condition typically with multifocal lymphatic manifestations that results in progressive osseous destruction. A lymphoscintigram study would show uptake.

Treatment:

• In symptomatic primary extradural meningiomas, total tumor removal with a wide surgical resection followed by cranial reconstruction should be considered. Adjuvant radiotherapy or chemotherapy can be considered if there is residual disease or WHO grade II/III classification.
• In the context of our patient, with the atypical radiologic appearance of the lesion and potential for an underlying aggressive lesion, surgical resection was performed, which confirmed the diagnosis.