Soft Tissue Masses: How Ultrasound Differentiates a Lipoma from a Suspicious Tumor

Dr. Segnini
Dec 16, 2025
3 min read

Updated: Dec 22, 2025

The “Lump” Under the Skin

The appearance of a lump or mass under the skin is a common concern. Fortunately, the vast majority of these masses in soft tissues (muscle, fat, skin, connective tissue) are benign. Ultrasound is the imaging technique of choice for their initial evaluation, as it offers a real-time, dynamic, and detailed window to characterize them without using radiation.

The Crucial Role of Ultrasound: Beyond “Seeing a Lump”

Ultrasound not only confirms the presence of the mass, but also answers key questions for diagnosis:

Where exactly is it? (In the skin, in the subcutaneous fat, inside a muscle).
What is its internal structure? (Solid, cystic, mixed).
What are its edges like? (Well-defined or infiltrating).
Does it have vascularization? (Internal blood flow, evaluated with Doppler).
How does it behave when moved? (Dynamics in relation to adjacent muscle or tendon).

Gallery of Common Diagnoses: The Ultrasound Signature

1. Typical Benign Lesions:

Lipoma:

Appearance: Solid, well-defined, hyperechoic mass (bright gray-white on the screen) compared to muscle.
Key feature: Usually isoechoic or slightly hyperechoic compared to the surrounding subcutaneous fat. May have fine internal streaks and is highly compressible with the transducer. Generally no vascularization on Doppler.
Why it is not a concern: Slow growth, sharp edges, and homogeneous fatty composition.

Epidermoid Cyst / Sebaceous Cyst:

Appearance: Rounded, anechoic (black) lesion or with fine echogenic content if infected.
Key feature: Posterior acoustic enhancement (bright area behind the cyst, like a “shadow echo”). May show an echogenic point corresponding to the pore connecting to the skin.
Dynamics: Clearly cystic, with no solids inside.

Synovial Cyst/Ganglion:

Appearance: Cystic structure (anechoic or with debris) arising from a joint or tendon sheath.
Key feature: Often multilobulated, with a “neck” connecting to the joint capsule. Classic in the wrist and back of the hand.

Hematoma / Seroma:

Appearance: Fluid collection with variable appearance depending on time (acute, subacute, organized).
Key feature: Recent history of trauma or surgery. No internal vascularization. May have fluid-debris levels.

2. Findings that Raise Suspicion (Possible Tumor):

Hyperechoic solid mass: A lesion darker than the surrounding muscle is a red flag.
Irregular or poorly defined edges: Absence of a clear capsule, with infiltrating extensions.
Internal heterogeneity: Mixed areas of different echogenicity, with possible areas of necrosis (fluid within the solid).
Doppler hypervascularization: Presence of abundant and chaotic blood flow within the solid mass. A typical lipoma does not have this pattern.
Rapid growth (in evolutionary control): Increase in size in weeks or months.
Deep location: Masses within the muscle plane or deeper have a higher risk than superficial subcutaneous ones.

The Ultrasound Protocol: From Image to Action

Systematic examination: The mass is evaluated in two planes, measured, and its relationship to anatomical planes is described.
Characterization: It is determined whether it is cystic, solid, or mixed, and the aforementioned signs are evaluated.
Color/Power Doppler: To assess vascularization, a crucial piece of information.
Structured report: The radiologist issues a report describing the findings and offering a diagnostic impression (e.g., “Ultrasound characteristics consistent with lipoma” or “Solid hypoechoic vascularized mass, evaluation with MRI/biopsy recommended”).
Referral: Depending on the level of suspicion, the patient is referred to Traumatology/Oncology for monitoring, magnetic resonance imaging (MRI)—which offers better soft tissue contrast—or ultrasound-guided biopsy for definitive histological diagnosis.

Conclusion: Peace of Mind and Accuracy

Ultrasound is an exceptional ally in the approach to soft tissue masses. In most cases, it can offer a definitive diagnosis or a high probability of benignity (as in lipomas or cysts), providing peace of mind to the patient and avoiding unnecessary studies. When it identifies alarming characteristics, it accurately and safely guides the next diagnostic steps, ensuring that potentially serious lesions do not go unnoticed. “Not all lumps are the same, and ultrasound holds the key to distinguishing between them.”