Epidermoid

Findings

Figure 1 (T1 axial) and Figure 2 (T1 post-gadolinium contrast): Lesion is slightly hyperintense to but near CSF intensity. It arises inferior to the fourth ventricle with mass effect on the adjacent cerebellum and brainstem and encroaches on the foramen magnum. Post-contrast imaging of the mass demonstrates no appreciable enhancement.
Figure 3 (T2 axial): The lesion is hyperintense on T2-weighted imaging and is of CSF intensity.
Figure 4 and Figure 5 (FLAIR): The lesion shows some mixed signal on FLAIR imaging.
Figure 6 The lesion demonstrates restricted diffusion on diffusion-weighted imaging.


Diagnosis: Epidermoid


Epidermoids compose approximately 0.2%-1.8% of all intracranial tumors. Epidermoid is a congenital CSF-like mass that is also known as a congenital epidermal inclusion cyst. This mass of epithelium arises from inclusion of ectodermal rest cells during neural tube closure at the third to fifth week of embryogenesis. They are slow-growing, well-circumscribed, smooth or lobulated lesions. Histologically, they have an internal layer of stratified squamous epithelium with a whitish fibrous capsule; given these features, epidermoids are often called a pearly tumor. Keratin and cholesterol crystals are identified within them.

Epidermoids grow slowly, and do not present until ages 20-60, peak age 40, in men and women equally. They typically present with headache or neuropathy (cranial nerves V, VII, VIII most commonly involved), and symptoms depend upon location. Most present in an intradural location, and almost half present in the cerebellopontine angle (40%-50%). Epidermoids are the third most common cerebellopontine angle mass after vestibular schwannoma and meningioma. As in this case, epidermoids may present around the fourth ventricle (17%). They may also present in the parasellar/middle cranial fossa (10%-15%), and 10% may present in an extradural location within the skull or spine.

On CT, epidermoids are typically low density and expand to fill the CSF space, insinuating around normal brain structures and appearing similar to arachnoid cysts. Epidermoids demonstrate variable appearance on T1-weighted imaging, based upon protein and lipid content, and they do not enhance after contrast administration.

Since epidermoids, like arachnoid cysts, may be similar to CSF signal on T1- and T2-weighted imaging, diffusion imaging is useful to differentiate these entities. Since arachnoid cyst contains CSF, and CSF does not present restriction to the diffusion in any direction, arachnoid cyst is dark on diffusion. In contrast, since epidermoids (like cholesteatomas) are composed of epithelial cells that grow in layers in an organized spatial organization, they present with markedly restricted diffusion and bright signal on diffusion. In addition, epidermoids are characteristically brighter on FLAIR imaging because of incomplete nulling to suggest a solid tumor (arachnoid cysts are dark like CSF).

Treatment includes microsurgical resection. Since epidermoids often insinuate through adjacent normal brain structures, surgery is often complicated and recurrence is common if the mass is not completely removed. Subarachnoid dissemination of contents and malignant degeneration to squamous cell carcinoma are rare.