KEY FACTS
Terminology
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Slow-growing subdermal cyst that contains keratin and is lined by stratified squamous epithelium
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Does not contain dermal elements (dermoid cyst)
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Imaging
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Ultrasound appearances depend on maturation of cyst and amount and compactness of keratin
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Most epidermoids are avascular with thin, hypoechoic walls and posterior acoustic enhancement
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Most are heterogeneous and mildly hyperechoic
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Can appear homogeneous (“pseudotestis”)
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Well-circumscribed, ovoid (80%), lobulated (19%), or tubular (1%) configuration
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Linear, hyperreflective, disc-like areas represent layered keratin aggregates ± cholesterol deposition
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Hypoechoic tract extending from superficial wall to dermis (10%) (punctum)
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Hypoechoic, small, cyst-like spaces
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Contained rupture → lobulated configuration of cyst
Top Differential Diagnoses
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Nerve sheath tumor, soft tissue abscess, lipoma
Clinical Issues
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Occur on face, scalp, neck, trunk, back
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Incidental or firm, nontender mass
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Complications include superinfection or rupture
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Can be electively surgically removed
Scanning Tips
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Internal debris, when present, is mobile and “swirls” with compression
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Assess periphery for vascularity indicating cyst rupture
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Ill-defined margin and irregular shape can also indicate rupture
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Look for layering hypoechoic and hyperechoic rings
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Look for cutaneous punctum (tract)
