An atypical intradiploic epidermoid cyst, radiological findings, and surgical management.

Background: Epidermoid cysts are benign, slow-growing, and extra-axial lesions that typically develop between the brain structures. Intradiploic and intra-axial involvement is exceptionally rare. We present a case of an intradiploic epidermoid cyst with distinct clinical and radiological features. This case highlights an unusual location and radiological presentation of an epidermoid cyst located within the skull base and may be misdiagnosed as other pathologies.

Case description: A 44-year-old male presented with a dull, aching frontal headache and slowly growing right frontal swelling, worsening over several months. On examination, the patient had a hard, non-tender, non-mobile right frontal swelling above the right orbit, with an unremarkable neurological examination. Computed tomography brain revealed an extra-axial hypodense lesion in the right frontal region, causing bony thinning and focal defects in the roof of the right orbit and the zygomatic process of the frontal bone. A mild mass effect on the superior rectus muscle and adjacent brain parenchyma was noted, with the left midline shift and effaced sulci. Magnetic resonance imaging (MRI) brain showed heterogeneous signals in both T1 and T2 as well as a central area of restricted diffusion in diffusion-weighted imaging, the blood products indicated by susceptibility-weighted imaging blooming artifacts, which were suggestive of hemorrhage. The radiological differential diagnosis included an epidermoid cyst, arachnoid cyst, dermoid cyst, abscesses, metastasis, and an aneurysmal bone cyst. The patient underwent a right frontal craniotomy and excision of the cyst. Intraoperative gross inspection and histopathological analysis confirmed the diagnosis of an intradiploic epidermoid cyst.

Conclusion: Epidermoid cysts occurring outside their usual locations are exceptionally rare, exhibiting atypical imaging characteristics, including unusual signal intensities on T1- and T2-weighted MRI sequences, along with the absence of the typical pattern of complete restricted diffusion. These findings may be indicative of a mixture of blood products and proteinaceous substances within the cyst. Subsequently, it may be misdiagnosed as other intracranial pathologies.