Epidermoid Cyst

KEY FACTS

Terminology

• Slow-growing subdermal cyst that contains keratin and is lined by stratified squamous epithelium

Cyst wall must not contain dermal elements

Imaging

• US appearances depend on maturation of cyst and amount and compactness of keratin

± presence of cyst rupture and degree of cyst collapse
• Well-circumscribed ovoid (80%), lobulated (19%), or tubular (1%) configuration
• Increased acoustic transmission (96%)
• Most are heterogeneous and mildly hyperechoic
• Some homogeneous with pseudotestis appearance
• Linear hyperreflective disc-like areas represent layered keratin aggregates ± cholesterol deposition
• Cleft-like or filiform anechoic or hypoechoic areas are due to cyst-like spaces lacking keratin
• Thin, hypoechoic cyst wall

Hypoechoic tract extending from superficial wall to dermis (10%) (punctum)
• Contained rupture → lobulated configuration of cyst
• Most epidermoid cysts are avascular

Peripheral and pericystic vascularity is feature of cyst rupture and inflammation
• Sensitivity/specificity of US in making diagnosis of epidermoid cyst is 80% and 95%, respectively

Top Differential Diagnoses

• Lipoma, soft tissue abscess, slow-flow vascular malformation, nerve sheath tumor

Diagnostic Checklist

• Characteristic US appearances in most cases
• Consider localized leakage of cyst content if inflammation is present around epidermoid cyst

TERMINOLOGY

Synonyms

• Epidermal inclusion cyst, epidermal cyst, infundibular cyst, sebaceous cyst

Definitions

• Slow-growing subdermal cyst that contains keratin and is lined by stratified squamous epithelium

Most cysts are due to ductal blockage of hair follicles with downward rupture of obstructed pilosebaceous unit

Some cysts are due to traumatic or embryological subdermal sequestration of epithelial tissue

– Histologically similar and hence grouped under general term “epidermoid cyst”

Wall of cyst must not contain dermal elements

– Presence of dermal elements → designation as dermoid cyst

Epidermis = outer layers of skin

– Contains keratinocytes, corneocytes, and melanocytes

Dermis = inner layers of skin that support epidermis

– Contains hair follicles, nerves, sweat glands, small blood vessels, and sebaceous glands

IMAGING

General Features

• Best diagnostic clue

Well-circumscribed, cyst-like mass with thick, linear, seed- or disc-like hyperechoic foci, filiform anechoic areas, and dermal attachment
• Location

Predominately in those areas with most body hair

Most commonly seen in head and neck

Trunk and extremities less commonly found

– Most cysts occurring in relatively hairless areas, such as palm or sole, are implantation epidermoid cysts

– Trauma → implantation of epidermal elements to dermis or subcutis, which then survive, grow, and produce keratin

Can occur within tendon or bone
• Size

< 1 cm to > 10 cm, usually < 5 cm
• Morphology

Well-circumscribed subcutaneous or dermal mass

Radiographic Findings

• Soft tissue swelling if large size
• Rarely bony remodeling or lytic bone lesion with sclerotic border

CT Findings

• Noncalcified soft tissue attenuation mass in subdermal layer

Low-density, fluid-like attenuation due to high lipid content

– Rarely hyperdense due to hemorrhage, saponification, or high protein content
• No enhancement or minimal rim enhancement

MR Findings

• T1WI

Isointense to slightly hyperintense relative to muscle
• T2WI

Hyperintense signal with high or low signal intensity content

– Low-signal debris represents keratin aggregates

– High-signal debris represents cholesterol deposition

Content may be positionally dependent or layered concentrically

T2-hypointense rim due to stratified squamous epithelial wall
• T1WI C+

Absent to low level of central enhancement with enhancing capsule

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Apr 9, 2020 | Posted by admin in MUSCULOSKELETAL IMAGING | Comments Off

KEY FACTS

Terminology

TERMINOLOGY

Synonyms

IMAGING

General Features

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Epidermoid Cyst

Epidermoid Cyst

Imaging

Top Differential Diagnoses

Diagnostic Checklist

Definitions

Radiographic Findings

CT Findings

MR Findings

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Epidermoid Cyst

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