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)

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

Contained rupture → lobulated configuration of cyst

Nerve sheath tumor, soft tissue abscess, lipoma

Occur on face, scalp, neck, trunk, back

Incidental or firm, nontender mass

Complications include superinfection or rupture

Can be electively surgically removed

Internal debris, when present, is mobile and “swirls” with compression

Assess periphery for vascularity indicating cyst rupture

Ill-defined margin and irregular shape can also indicate rupture

Look for layering hypoechoic and hyperechoic rings

Look for cutaneous punctum (tract)