Atypical imaging features of posterior fossa's dermoid cyst: Case report and review of literature.

BACKGROUND: Intracranial dermoid cysts are uncommon lesions with characteristic imaging appearances. Symptomatic clinical presentation usually occurs in one of two ways: mass effect or rupture. Radiologically, dermoid cysts typically present as low density masses on computed tomography (CT) scan and are generally hyperintense on T1-weighted magnetic resonance imaging (MRI) sequences with variable signal on T2-weighted sequences. CASE DESCRIPTION: We present the case of a 35-year-old female presented with symptoms of increased intracranial pressure. Radiological investigations showed a cystic posterior fossa tumor that was not only hyperdense on CT scans but also hypointense on MRI T1-weighted images. The patient underwent a total-gross resection of an extra-parenchymal posterior fossa tumor. Pathologic examination of the specimen concluded to dermoid cyst.
CONCLUSION: Dermoid cyst of the posterior fossa is a benign lesion surgically treatable. Only an appropriate radiological diagnosis of this lesion would permit a well-targeted therapeutic approach.

INTRODUCTION

Dermoid cysts are rare congenital lesions as a result of developmental malformation, with the defect in gastrulation affecting the surface ectoderm and causing a secondary disruption of neural tube closure.[10] Computed tomography (CT) and magnetic resonance imaging (MRI) delineate many characteristic features of dermoid cysts. Their most common site is the posterior cranial fossa nearby the midline, when extradural, may arise at the anterior fontanelle.[36] However, some unusual radiological features may elude radiologists and surgeons to other differential diagnosis.

In this paper, we report a patient having fourth ventricle dermoid cyst with atypical radiololgical features.

CASE REPORT

We report the case of a 35-year-old female presented with headaches, vomiting, and gradually worsening blurred vision. Physical examination was unremarkable except for a grade 2 papillary edema on the fundi. Brain CT scan showed imaging a large median and paramedian spontaneous homogenous hyperdense lesion in the posterior fossa. Mild surrounding edema and mass effect on the fourth ventricle was associated provoking a triventricular hydrocephalus [Figure 1]. On MRI, the lesion appeared hypointense on T1-weighted images and extremely hypointense on T2-weighted images with a hyperintense central area on T1 and T2 without enhancement after injection of gadolinium [Figures 2–4]. A total resection of an extra-parenchymal tumor was performed through a midline suboccipital craniotomy. The lesion was cystic and nonhemorrhagic. After opening the thin cyst wall, a viscid and light brown substance containing few short dark hairs was removed. The patient had a good postoperative recovery and was discharged on the 5th-postoperative day. Histopathologic examination [Figure 5] revealed a stratified squamous epithelium with fatty cellular debris, few hair follicles, and scattered sebaceous glands. The diagnosis of dermoid cyst was retained.

Figure 1

Axial CT scan showing a hyper-attenuating lesion centered by an hypodensity of the posterior fossa

Figure 2

MRI on axial T1 weighted image revealing an hypointense and heterogeneous contrast enhancement of the solid component of the dermoid cyst

Figure 4

MRI on Sagittal T2 weighted images showing a hypointense signal of the lesion

Figure 5

Histopathological examination of the surgical specimen revealing a stratified squamous epithelium with fatty cellular debris, few hair follicles and scattered sebaceous glands consistent with the diagnosis of dermoid cyst

MRI on axial T2 weighted images showing a hypointense signal of the dermoid cyst

DISCUSSION

The intracranial dermoid cyst is a rare entity, accounting for 0.1–0.7% of all intracranial tumors.[14] Dermoid cysts are derived from the ectopic inclusion of epithelial cells during neural tube closure between the 3rd and 5th week of fetal development, so that the preferential location in the medial line of the posterior fossa and the spinal cord.[1113] They are slow growing lesions and may become quite large before becoming symptomatic. Rarely, they may rupture and present with meningitis.[313] Dermoid cysts are well-circumscribed lesions lined by stratified squamous epithelium. They contain thick, foul-smelling, yellowish material from the secretion of sebaceous glands, lipid metabolites, whorls of hair, calcifications, and desquamated epithelium.[5]

To our knowledge, nine cases have been reported about posterior fossa's dermoid cyst so far. All those cases are resumed in Table 1.

Table 1

Cases of posterior fossa's dermoid cyst reported in the literature

The appearance of the dermoid cyst in typical cases is easily recognizable in imaging. They are usually extremely hypodense on CT scan with a Hounsfield unit (HU) of −20 to −140, explained by their lipid content.[45] On MRI, dermoid cysts typically appear hyperintense on both T1-weighted and FLAIR sequences, with variable signal on T2-weighted sequences that ranges from hypointense to inhomogeneously hyperintense. There is usually associated neither vasogenic edema nor contrast enhancement. The presence of spontaneous hemorrhage is extremely rare.[3] On both CT and MRI images, fat density droplets may be seen throughout the subarachnoid space and inside the ventricular system if rupture of the cyst has occurred.

The unusual radiological finding in our patient was the hyperdensity on CT scan and the hyposignal on MRI T1-weighted images. The hyperdensity could be explained by the high protein content of the necrotic tissue within the lesion.[4] The hyposignal on T1-weighted images could be explained by the saponification of lipid or keratinized debris with secondary microcalcification in suspension.[12]

CONCLUSION

The dermoid cyst of the posterior fossa is a benign lesion surgically treatable. The appropriate radiological diagnosis of this lesion permits a well-targeted therapeutic approach. The radiological study is often unequivocal during this lesion. Nevertheless, the hyperdense aspect of the lesion on CT associated with hyposignal on T1-weighted images on MRI may make the diagnosis of a dermoid cyst more difficult.

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Conflicts of interest

There are no conflicts of interest.