Neuroradiology

Epidermoid tumour

congenital non-neoplastic inclusion cyst (see dermoid tumours) that arises from inclusion of ectodermal epithelial elements at the time of closure of the neural tube.

The epidermoid cyst has a thin capsule composed of a simple stratified cuboidal squamous epithelium, and is filled with a soft keratohyalin material resulting from the desquamation of the cyst wall. It has a cauliflower-like outer surface with a typical shiny mother-of-pearl appearance. It is usually located in the basal subarachnoid spaces, suprasellar cistern, fourth ventricle and cerebellopontine angle cistern, encasing vessels and nerves; an extradural location is uncommon and is usually in the diploic bone of the cranial vault.

At CT scan epidermoids appear as well-defined hypodense masses, with a CSF density in the typical subarachnoid location, frequently slightly displacing the adjacent nervous tissue. On MR they typically have long T1 and T2 signals (Fig.1). Diffusion-weighted images help in differential diagnosis with arachnoid cysts; epidermoids show high signal due to reduced diffusion coefficient of water within the tumour (Fig.2).

A different and rare entity in modern times is the acquired epidermoid cyst, caused by iatrogenic subarachnoid implantation of epidermal fragments during lumbar puncture which was performed in the past with nonstylet needles (see lumbar puncture sequelae in epidermoid cyst).

GS

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Fig.2

a, b. MR, T1-weighted images following gadolinium injection, sagittal (a) and axial (b) views. The tumour, located within the fourth ventricle, is markedly hypointense, almost as CSF and does not show any enhancement. Third and lateral ventricles are dilated. c. MR, axial T2-weighted image. Marked homogeneous hyperintensity of the lesion. d, e. MR, diffusion-weighted images on the sagittal and axial planes. Inhomogenous hyperintensity of the lesion due to reduced diffusion coefficient of water.
Epidermoid tumour, Fig.1 (a)
Epidermoid tumour, Fig.1 (b)
Epidermoid tumour, Fig.2 (a)
Epidermoid tumour, Fig.2 (b)
Epidermoid tumour, Fig.2 (c)
Epidermoid tumour, Fig.2 (d)
Epidermoid tumour, Fig.2 (e)