Soft Tissue Mass Evaluation on Ultrasound

Soft tissue masses are one of the most common referrals to the ultrasound department. The majority are benign — lipomas, ganglion cysts, epidermal inclusion cysts — but the job is to characterize each lesion carefully and identify the minority that warrant further workup.

Your Job When Evaluating Any Soft Tissue Mass

1. Confirm it's a real mass (not normal tissue, artifact, or lymph node)
2. Describe it systematically: size, depth, location, echogenicity, margins, internal structure, vascularity
3. Determine cystic vs solid
4. Identify imaging features that suggest a specific diagnosis
5. Flag features that concern for malignancy
6. Recommend next step if needed

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Cystic vs. Solid: The First Decision

Simple cyst criteria (all must be met):

• Anechoic
• Thin, imperceptible wall
• Posterior acoustic enhancement (PA enhancement)
• No internal vascularity

If all criteria are met → simple cyst. Management is conservative unless symptomatic.

Complex cyst: Meets some but not all criteria — has internal echoes, debris, septations, or a thickened wall. Requires characterization.

Solid mass: Echogenic or hypoechoic internal structure without PA enhancement. Needs full characterization and Doppler.

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Common Benign Masses

Lipoma

The most common soft tissue mass. A benign fatty tumor.

Sonographic appearance:

• Well-defined, oval or elongated mass
• Compressible on probe pressure
• Echogenicity: isoechoic or hyperechoic relative to subcutaneous fat (variable — can be hypoechoic)
• Thin echogenic capsule
• Parallel linear echogenic striations within (fat lobules)
• No significant internal vascularity — a few scattered internal vessels are acceptable, but hypervascular lipoma is a red flag
• Deep to skin, within or between muscle planes depending on depth

The tricky part: A lipoma in a subfascial (intramuscular) location is harder to characterize — these have a higher chance of being misclassified. Intramuscular lipomas should have MRI if:

• Depth: subfascial (deep to the investing fascia)
• Size > 5 cm
• Heterogeneous internal appearance
• Any vascularity

Simple subcutaneous lipoma: < 5 cm, homogeneous, no vascularity → no further imaging needed; clinical diagnosis with ultrasound confirmation.

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Ganglion Cyst

A ganglion is a cystic structure filled with thick, viscous gelatinous fluid, arising from a joint capsule or tendon sheath.

Location: Most commonly dorsal wrist, volar wrist, and foot/ankle.

Sonographic appearance:

• Anechoic or hypoechoic cyst (may have low-level internal echoes from viscous fluid)
• Round or lobulated
• Well-defined, thin wall
• Posterior acoustic enhancement
• No internal vascularity
• Often has a visible pedicle (stalk) connecting it to the adjacent joint or tendon sheath — follow the cyst to its origin

Important: If the ganglion is near the wrist, document proximity to the ulnar artery and median nerve — these can be entrapped or displaced.

Management: Often asymptomatic and observed. Aspiration is guided by ultrasound.

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Epidermal Inclusion Cyst (EIC)

A benign cyst containing keratin — arises from obstructed hair follicles or implanted skin cells (post-trauma, post-surgery).

Location: Skin and subcutaneous tissue — anywhere, but most common on the scalp, face, neck, trunk, and extremities.

Sonographic appearance:

• Well-defined, round to oval hypoechoic cyst in the dermis or subcutaneous tissue
• Often shows a punctum — small echogenic dot on the skin surface at the center of the cyst (the obstructed follicle opening)
• Internal echoes from keratinous debris (pseudo-solid appearance)
• Posterior acoustic enhancement despite internal echoes
• No vascularity internally (occasionally rim vascularity if inflamed)

Key: The punctum helps confirm EIC vs other cysts.

Infected EIC: Internal echoes increase, rim of peripheral vascularity, tenderness on probe pressure. Distinguish from abscess — abscess is usually more heterogeneous with frank purulent debris and surrounding hyperemia.

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Pilomatrixoma

A benign calcifying epithelial neoplasm — common in children and young adults, typically on the head, neck, and upper extremities.

Sonographic appearance:

• Subcutaneous mass with echogenic rim or internal calcifications — sometimes heavily calcified with posterior shadowing
• Heterogeneous internal architecture
• Small cystic components possible

Often confused with EIC clinically. The calcifications are the clue.

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Hemangioma and Vascular Malformations

Hemangioma: Benign vascular tumor of infancy. Well-defined, lobulated, heterogeneous mass with marked internal vascularity (often hypervascular on color Doppler). Usually superficial.

Vascular malformations:

• Venous malformation: Compressible, heterogeneous mass; may contain phleboliths (round echogenic foci with posterior shadowing — classic). Flow on Doppler is low-flow or absent.
• Arteriovenous malformation: High-flow lesion — visible flow in multiple channels on Doppler; arterial waveforms in otherwise venous locations.

Document vascularity, compressibility, and presence of phleboliths.

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Features That Should Raise Concern

No single ultrasound feature reliably distinguishes benign from malignant soft tissue masses — MRI is required for concerning lesions. But these features warrant further evaluation:

Feature	Concern
Depth: subfascial	Higher malignancy risk than subcutaneous
Size > 5 cm	Malignant soft tissue tumors tend to be large
Heterogeneous internal architecture	Necrosis, hemorrhage
Marked internal vascularity (hypervascular)	Malignant tumors often hyperemic
Irregular, ill-defined margins	Infiltrative growth
Rapid growth on serial imaging	Suspicious
Invasion of adjacent structures	Malignant

The "rule of twos" (informal teaching): A soft tissue mass that is > 5 cm OR subfascial OR has irregular margins should be evaluated with MRI before any biopsy.

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Documenting Any Soft Tissue Mass

Minimum documentation:

1. Location (anatomic region, depth — subcutaneous vs fascial plane vs intramuscular)
2. Size (three dimensions)
3. Echogenicity and internal character (cystic, solid, complex)
4. Margins (well-defined vs irregular)
5. Posterior acoustic features (enhancement, attenuation)
6. Vascularity (hypervascular, avascular, rim only)
7. Relationship to adjacent structures (tendons, vessels, nerves, bone)
8. Compressibility

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Reporting Language

Lipoma: "4.1 × 1.8 × 2.5 cm subcutaneous, well-defined, ovoid echogenic mass in the posterior left upper arm, parallel to the skin surface. The mass is compressible, hyperechoic relative to adjacent subcutaneous fat with internal linear echogenic striations. No significant internal vascularity. Findings consistent with a lipoma. No further imaging is required."

Ganglion cyst: "1.4 × 0.9 × 1.1 cm anechoic lobulated structure arising from the dorsal aspect of the wrist joint, with posterior acoustic enhancement and no internal vascularity. A connecting pedicle to the scapholunate joint is identified. Findings consistent with a ganglion cyst."

Concerning mass: "6.2 × 4.8 × 5.0 cm heterogeneous subfascial mass in the posterior right thigh. The mass has ill-defined margins, heterogeneous internal architecture with both solid and cystic components, and moderate internal vascularity. These features are indeterminate; MRI of the right thigh with and without contrast is recommended for further characterization."

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SonoBuddy's MSK and small parts protocols include soft tissue, lymph node, and joint evaluation guides.