Ultrasound of Epidermal Inclusion Cyst

In this radiology lecture, we review the ultrasound appearance of epidermal inclusion cyst!

Key teaching points include:

Epidermal inclusion cyst is the most common cutaneous cyst.
Can occur anywhere: Head, neck, trunk, extremities.
Benign, keratin-containing cyst lined by a wall of stratified squamous epithelium.
On ultrasound, appears as a well-circumscribed, round to oval mass with broad (50%) contact with dermis, nonvascular and with posterior acoustic enhancement.
Hypoechoic to minimally hyperechoic with internal linear echogenic and anechoic debris = “Pseudotestis.”
Presence of a focal hypoechoic tract extending towards epidermis adds specificity = “Submarine sign.” May see overlying punctum on skin surface = Small, dark-colored opening.
Epidermal inclusion cysts are different from sebaceous cysts. Sebaceous cysts originate from sebaceous glands, contain sebum and are less common.
Epidermal inclusion cysts contain keratin, not sebum, but are often incorrectly referred to as sebaceous cysts.
Epidermal inclusion cyst vs. epidermoid cyst. Epidermoid cyst is a non-neoplastic cyst lined only by squamous epithelium. Epidermal inclusion cyst is a specific type of epidermoid cyst caused by implantation of epidermal elements in the dermis. All epidermal inclusion cysts are epidermoid cysts, but not all epidermoid cysts are epidermal inclusion cysts.
Can become ruptured or infected: Ill-defined or lobular margins, internal blood flow, peri-lesional soft tissue inflammation, adjacent fat focally hyperechoic or hyperemic. DDx for complicated epidermal inclusion cyst: Neurogenic tumors and other neoplasms.
Uncomplicated cysts typically do not require treatment, but if infected may require I&D or excision. Growing cysts may also require excision.
Rare (1%) malignant degeneration to squamous cell carcinoma, less commonly basal cell carcinoma.

References:
Jacobson JA, Middleton WD, Allison SJ, et al. Ultrasonography of Superficial Soft-Tissue Masses: Society of Radiologists in Ultrasound Consensus Conference Statement. Radiology 2022; 304:18-30. https://pubs.rsna.org/doi/full/10.1148/radiol.211101

To learn more about the Samsung RS85 Prestige ultrasound system, please visit: https://www.bostonimaging.com/rs85-prestige-ultrasound-system-4

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