Epidermoid tumor of the anterior interhemispheric fissure

Clinical History

A 12-year-old girl presented at the Neurology department complaining of long standing headache.

Imaging Findings

A 12-year-old girl presented to the Neurology department complaining of long standing headache. Her general and neurological examination was normal. Routine blood investigations were within normal limits. MRI examination of the brain was performed using T1, T2 weighted standard spin echo sequences, FLAIR (Fluid Attenuating Inversion Recovery) and DW (diffusion weighted) sequences. MR imaging revealed an ovoid well-circumscribed and well-defined extra-axial mass lesion in the anterior interhemispheric fissure. The mass demonstrated hypointense signal on the T1-w images (Fig. 1 and 3). On T2-w images it appeared markedy hyperintense, demonstrating isointense signal to CSF (Fig. 2). On FLAIR images it appeared heterogeneous and was relatively higher in signal relative to the CSF (Fig. 4). The mass demonstrated restricted diffusion, appearing markedly hyperintense on DW images (Fig. 5), with relatively low signal on corresponding ADC maps (Fig. 6). Based upon the MR imaging findings diagnostic possibility of an epidermoid tumour was considered. Intra-operatively a lobulated well defined extra-axial mass was seen partially adherant to the cribriform plate. Near-total resection of the mass was done and histopathological examination confirmed it to be an epidermoid tumour.

Discussion

Epidermoid and dermoid tumours are slow-growing, benign, ectodermal inclusion cysts. It is the location and lesion characteristics which distinguish epidermoid from dermoid cysts. Both dermoid and epidermoid cysts result from ectodermal inclusions, however cyst lining in dermoids unlike epidermoids further differentiates to include dermal appendages such as hair follicles, sebaceous glands, and sweat glands. Epidermoid tumours contain solid crystalline cholesterol, whereas the dermoid cysts have liquid cholesterol. Classically epidermoid are considered to be “off-midline” tumours, usually located on one side of the brain or skull, while dermoid tumours are thought to be midline cysts. Epidermoid tumour located in the midline, is a rare occurrence, with limited reports in the literature. We present here an unusual case of an anterior interhemispheric epidermoid which was found to be closely adherent to the floor of anterior cranial fossa (cribrifom plate) intra-operatively.

Epidermoid tumour accounts for 0.2–1.8% of all brain tumours. These are benign, congenital developmental lesions which arise from ectodermal inclusion during neural tube closure in the 3rd-5th week of embryogenesis. Although medical literature uses the term "cyst" or "tumour" interchangeably, “tumour” appears more appropriate as these lesions are solid in nature. These tumours are well marginated and thin walled, lined by stratified squamous epithelium. They tend to gradually enlarge as epithelial cells desquamate resulting in accumulation of keratin and cholesterol crystals. Owing to their contents and a whitish fibrous capsule, these tumours have been referred as "pearly tumour/ mother of pearl/ beautiful tumour”. The most common locations are within the cerebellopontine (CP) angle cistern (40-50%), supra-and-parasellar region (7% each), and middle cranial fossa. Of all CP angle masses, epidermoids are the third most common after vestibular schwannoma and meningioma. Ten percent of epidermoids are extradural, mostly intradiploic. Intra-axial epidermoid tumours are highly uncommon. The fourth ventricle is the most common site. Rarely, these tumours occur in the cerebral parenchyma or the brainstem. Anterior interhemispheric fissure epidermoid cysts are rarer still, with limited case reports in the literature. The typical imaging appearance of an epidermoid is a well-circumscribed, non-enhancing extra-axial mass, insinuating into CSF spaces. It may encase vessels and nerves. It is seen as a low attenuation mass on CT-scan. MRI is the preferred imaging modality. On MRI, epidermoids show hypo- to isointense signal on T1w, hyperintense on T2w images as well as diffusion-weighted (DW) imaging. On FLAIR sequence these appear heterogeneous and show partial suppression, demonstrating higher signal relative to CSF. FLAIR and DW sequences have proven to be invaluable for confirming solid nature of epidermoid and differentiating it from an arachnoid cyst which is the closest differential. Unlike epidermoids which are solid lesions, CSF-containing arachnoid cysts get completely suppressed on FLAIR images and do not show restriction on DW-imaging. Epidermoids classically do not show post contrast enhancement, however mild peripheral rim enhancement has been reported in some cases. Treatment is surgical and often has a favourable prognosis. Rarely, malignant transformation into a squamous-cell-carcinoma may be encountered, which should be suspected if the lesion shows a newer contrast enhancement and a rapid growth rate.

Differential Diagnosis List

Final Diagnosis

Epidermoid tumor of the interhemispheric fissure

URL:	https://eurorad.org/case/8470
DOI:	10.1594/EURORAD/CASE.8470
ISSN:	1563-4086