Diagnosis

Epidermoid. Epidermoids are divided into black epidermoids and white epidermoids depending on their CT and MR characteristics. This case represents a unique combination of imaging features for a surgically proven epidermoid, which demonstrated high density on CT and high signal on T1-weighted MRI.

Discussion

Epidermoids are referred to as "pearly tumors" and occasionally as congenital or primary cholesteatomas. Favored locations include the cerebellopontine angle and suprasellar and prepontine cisterns. When in the basal cisterns, they often surround or displace major arteries. Other locations include sella turcica, middle cranial fossa, intraventricular, cerebellar hemispheres, and diploic space.^1-4 They are of ectodermal origin resulting from inclusion of epithelial elements at the time of neural tube closure.^1,5 Epidermoids are composed of stratified squamous epithelium surrounding cholesterol crystals and debris from desquamated keratin from the epithelial cyst lin-^2,4,6 Progressive desquamation leads to slow growth. There are two types of epidermoids, both of which are extraaxial well-circumscribed lesions. The "black epidermoid" has CT attenuation of water or fat with hypodensity secondary to cerebral spinal fluid, lipid, and cholesterol content. Peripheral calcifications are seen occasionally. Magnetic resonance imaging typically demonstrates T1 and T2 signal intensity which approximates that of cerebral spinal fluid; that is, there is hypointensity on T1 and hyperintensity on T2 relative to adjacent brain parenchyma. The low T1 signal may be secondary to the presence of cholesterol in the solid rather than liquid state or perhaps secondary to inclusion of cellular debris and cerebral spinal fluid. Black epidermoids have a low lipid content and no triglycerides or fatty acids.^5-7 The "white epidermoids" have high T1 signal on MR with negative Hounsfield units on CT secondary to their high cholesterol, lipid, triglyceride, and fatty acid content.^4-6 Epidermoids rarely are hyperdense on noncontrast CT. Proposed theories for the high CT density include saponification of debris to calcium soaps, calcification of keratin debris, elevated protein content, fibrosis, deposition of ferrocalcium or deposition of hemoglobin or hemosiderin secondary to a prior bleed.^1,7 Signal intensity on T2 typically is hyperintense for "black epidermoids" and isointense for "white epidermoids." Hypointensity on T2 is very rare and may be caused by calcification, low hydration, viscous secretions, or the paramagnetic effect of iron containing pigment.² Our case demonstrates CT hyperdensity with T1 hyperintensity and T2 hypointensity. We were able to find only 13 reported cases of epidermoids that were hyperdense on noncontrast CT.¹ However, we were unable to find any reported cases of an epidermoid that demonstrated high density on CT as well as high signal on T1-weighted MRI. One theory for this unique constellation of imaging findings would be recent hemorrhage into the epidermoid with the presence of intracellular methemoglobin. This appearance should not be mistaken for a subarachnoid hemorrhage.

1. Gao P, Osborn AG, Smirniotopoulos JG, et al: Epidermoid tumor of the cerebellopontine angle. AJNR 13:863-872, 1992.
2. Gualdi GF, Dibiasi C, Trasimeni G, et al: Unusual MR and CT appearance of an epidermoid tumor. AJNR 12:771-772, 1991.
3. Steffey DJ, DeFilipp GJ, Spera T, et al: MR imaging of primary epidermoid tumors. JCAT 12(3):438-440, 1988.
4. Tampieri D, Melanson D, Ethier R: MR imaging of epidermoid cysts. AJNR 10:351-356, 1989.
5. Horowitz BL, Chari MV, James R, et al: MR of intracranial epidermoid tumors: Correlation of an vivo imaging with in vitro ¹³C spectroscopy. AJNR 11:299-302, 1990.
6. Wagle WA, Jaufmann B, Mincy JE: Magnetic resonance imaging of fourth ventricular epidermoid tumors. Arch Neurol 48:438-440, 1991.
7. Tekkok IH, Cataltepe O, Saglam S: Dense epidermoid of the cerebellopontine angle. Neuroradiology 33:255-257, 1991.