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 Click the YouTube Community tab or follow on social media for bonus teaching material posted throughout the week! Spotify: https://bit.ly/spotify-rhq Instagram: https://www.instagram.com/radiologistHQ/ Facebook: https://www.facebook.com/radiologistHeadQuarters/ Twitter: https://twitter.com/radiologistHQ Reddit: https://www.reddit.com/user/radiologistHQ/