A slightly lobulated markedly hyperintense mass is present in the right
parasellar region. The lateral lobule of the mass is markedly hyperintense.
The medial lobule
demonstrates considerable heterogeneity of its intensity appearing
predominently hypointense suggestive of fatty material.
Intracranially these are rare tumors compared to epidermoid cysts. As with
epidermoid cysts, they arise from inclusion of ectodermoid elements in the
neural groove at its time of closure. Not uncommonly there is a persistent
defect in the overlying skin, with a sinus tract extending into the
Clinically, they usually present
in the third decade and are most commonly located in the posterior fossa
in the midline but may occur in the cisterns about the sella turcica and
elsewhere. They may also have an intraventricular location arising within
the cisterns of the tela choroidea in the lateral, third, or fourth
ventricular regions. Microscopically, these cysts may contain elements
from all layers of the skin. Much of the wall of the cysts may be lined
(as in epidermoid tumors) by stratified squamous epithelium supported by
an outer collagen layer. More solid portions of the tumor may contain hair
follicles along with sebaceous, sweat, and apocrine glands. Calcification
may develop in the portion of the walls, and there may be bone and
cartilage within some of the cysts.
Dermoid cysts typically demonstrate marked hyperintensity on short TR/TE
MR images due to their fatty content, which consists of triglycerides and
unsaturated fatty acids. On long TR images, the cysts usually show
hypointensity, particularly on long TE images, again due to the presence
of the fatty material. The hypointensity of the mass may be throughout or
within one or more loculations within the lesion. Other portions of the
cysts may demonstrate a pattern consisting of inhomogeneous hyperintensity,
similarto epidermoid tumors. The cysts may rupture into the subarachnoid
space or, for those arising in the tela choroidea, intraventricularly.
Subarachnoid rupture will demonstrate droplets and streaks of high
intensity within the subarachnoid cisterns about the tumor and possibly
more distally in the brain. Within the ventricles a fat-fluid level will
develop in the anterior superior portions. A chemical shift artifact will
frequently be projected into the lesion on long TR sequences. On short TR
sequences a high-intensity fluid level will be present anterior to the
hypointensity of CSF, whereas on long TR sequences an intermediate and
low-intensity fluid collection will be observed anterior to the high
intensity of the CSF.
Ashok Raghavan, Manipal Hospital, Bangalore