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Findings: MRI obtained with T1 weighted and multiecho axial images was obtained. Thereafter gadolinium was administered and T1 weighted axial and coronal images were obtained.
All images demonstrate a large extra-axial mass in the mesial right frontal lobe, contiguous with the interhemispheric fissure. T1 and multiecho images demonstrate a heterogenous signal within the mass. The mass is predominantly bright, particularly around the periphery with the center demonstrating some low signal. Around the anterior aspect of the lesion a chemical shift artifact is appreciated. The lesion produces a minimal mass effect on the genu of the corpus callosum and the frontal horns of the lateral ventricles. There is no significant enhancement following gadolinium administration.
Differential Diagnosis: Lesion showing a short T1 and T2 relaxation (i.e. high signal intensity on both) include:
Discussion: Dermoids and the closely related epidermoids are extremely rare intracranial tumors. Incidence of (0.1% to 0.7%). There are cystic structures with a dense collagenous wall, lined by stratified squamous epithelium. The cavities are filled with acellular, keratinaceous debris, particles of hair, and secretory material. Dermoids grow by this exfoliation of debris and deposition of hair and secretory material into the cyst cavity.
Both dermoid and epidermoids are the result of ectodermal inclusions during the closure of the neural groove between the third and fourth weeks of embryogenesis. However, unlike an epidermoid, in order for the ectodermal elements to form a dermoid, they must be in direct contact with the mesodermal tissue. Dermoids are midline lesions situated in the suprasellar, subfrontal, posterior fossa, or peripineal regions.
They tend to become symptomatic within the first three decades and symptoms depend on the location, tumor depth, and the size of the mass. Further symptomatology may result from a rupture of the dermoid. The spread of its contents into the ventricles with subarachnoid spaces may result in aseptic meningitis, seizure, and transient cerebral ischemia secondary to vasospasm.
MRI of these tumors appears hyperintense on T1 weighted images due to their fat content. However, not all the tumors demonstrate homogeneity as the cyst may also contain hair and other debris. It is also important to note that the dermoids have little vascularity and therefore do not contrast enhance. After rupture, the fatty contents entering the ventricles or subarachnoid space will also appear bright on T1 weighted images.
References:
Tumors by A.A. Williams in Craniocomputed
Tomography: A Comprehensive Text, A.A. Williams and Victor M.
Haughton, DCV Mosby Company, St. Louis, 1985, Pg 214.
Wilms G, Casselman J, Demaerel C: Neuroradiology 33:149-151, 1991.
Pomeranz, S: Craniospinal Magnetic Resonance Imaging, W.B.
Saunders, Philadelphia, Pgs 210, 237-238, 1989.
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