Teaching Case

16-year-old female patient presented with swelling over the left upper eyelid on the lateral aspect. Swelling was present since birth and showed slight increase in size over time. No associated vision problems were noted.

MR imaging revealed a well-defined round to oval-shaped lesion with smooth margins in the intra-cutaneous plane at the level of the lateral canthus of the left eye in preseptal location. It appears hyperintense (Fig. 1a, c - red arrows) with internal low-signal areas (Fig. 1b - blue arrows) on T2 sequence and hypointense (Figure 1d - red arrow) on T1 sequence. It also shows restriction (Fig. 1e - red arrow) on diffusion-weighted imaging and low ADC values (Fig. 1f - red arrow). On T1 post-contrast imaging (Fig. 2b - red arrow), no enhancement is noted. No evidence of intra-orbital or intracranial extension is noted. No evidence of surrounding fat stranding is noted. Further ultrasonographic evaluation of the lesion showed a well-defined hypoechoic cystic lesion (Fig. 3a - red arrow) at the lateral canthus of the left eye with internal inhomogeneity due to keratin debris. The lesion shows no internal vascularity (Fig. 3b) or calcifications. The above findings suggest diagnosis of epidermoid cyst.

Epidermal cysts or epidermal inclusion cysts or epidermoid cysts are common benign cutaneous lesions. They are usually asymptomatic and may be detected incidentally as firm non-tender masses. They are most commonly located in subcutaneous plane on the scalp, face, neck, trunk and back [1]. Malignant transformation is a rare occurrence. Epidermoid cysts are uniloculated lesions without septations. Histologically, they are accumulations of keratin, protein, cholesterol, and cell membrane lipids with a surrounding layer of squamous cell lining. They are slow-growing lesions. Proposed causative factors include squamous transformation of the columnar epithelial cells, downward migration of the epidermal cells with inflammation after obstruction of the hair follicle, and implantation of epidermal cells within the dermis following trauma [2]. MR imaging shows these lesions as hypo to isointense on T1-weighted images, hyperintense on T2-weighted images with tiny internal low-signal areas and restriction on diffusion-weighted imaging with low ADC values. Epidermoid cysts contain keratin, debris and cholesterol which restrict the diffusion of water molecules. On contrast-enhanced T1-weighted images, these lesions show no enhancement except for a thin peripheral rim enhancement in some cases. Typical ultrasound features include well-circumscribed, mildly echogenic lesion with posterior acoustic enhancement, internal linear echogenic reflections and no vascularity on colour Doppler imaging. The internal low-signal areas in T2-weighted MR imaging and internal linear echogenic reflections on ultrasound depict internal keratin debris suggesting diagnosis of epidermoid cyst [2]. The differential diagnosis of unruptured epidermal cyst includes dermoid cyst, lymphangioma, solid tumours like neurofibroma, myxoid tumours and dermatofibrosarcoma protuberans and solid-cystic tumours like nodular hidradenoma; all of which may appear bright on T2-weighted images. Tiny hypointense areas in epidermal cysts on T2-weighted images can be used to differentiate epidermal cysts from other fluid cysts. Dermoid cysts typically show T1-hyperintensity and fat suppression that distinguishes it from epidermoid cyst. Contrast-enhanced MR images show variable central enhancement within the solid masses which distinguishes solid tumours from epidermoid cysts. Moreover, solid tumours including nodular hidradenoma show internal vascularity differentiating them from epidermoid cysts. A ruptured epidermal cyst shows thick and irregular peripheral rim enhancement, surrounding soft-tissue reactions which may simulate infectious or neoplastic lesions on MRI and should be included in the differential diagnosis list [3]. These lesions usually do not require any treatment. Excision is required only if infection or rapid increase in size of the lesion is noted.
Teaching point: The characteristic MRI and sonographic findings of epidermal inclusion cysts lead to accurate diagnosis.