Ultrasound can suggest a fatty mass by echogenicity and compressibility; it cannot prove fat and cannot exclude liposarcoma.
Orient first
- A lipoma is typically compressible, isoechoic or mildly hyperechoic to subcutaneous fat, and avascular. None of those features proves fat.
- Thick internal septa, nodular hypoechoic foci and internal Doppler flow are the sonographic warnings — they are reasons to stop calling it a simple lipoma, not a diagnosis of sarcoma.
- Ultrasound cannot measure Hounsfield units and cannot run fat saturation, so the honest next sentence is the name of the confirming study.
Acquire the study
- High-frequency LINEAR probe; compare with adjacent subcutaneous fat at the SAME depth and gain.
- Colour Doppler with low-flow settings before concluding the mass is avascular.
- State whether the mass is subcutaneous or intramuscular — that single word decides MRI versus leave-alone.
The manoeuvre
- Measure in three planes and state the compartment: subcutaneous versus intramuscular.
- Compare echogenicity with adjacent fat at the same depth; note compressibility under the probe.
- Look for thick septa, nodular non-fatty foci and internal colour Doppler flow.
- If any of those are present, or if the mass is deep, name MRI as the next study rather than concluding lipoma.
What confirms it
- A well-circumscribed, purely fatty mass with only thin septa and no enhancement of nodules is a lipoma.
- Thick septa, nodular non-fatty foci or enhancement within the fat make well-differentiated liposarcoma the leading concern and need MRI if not already done, then histology.
What licenses you to exclude it
- ⚠️ ULTRASOUND AND CT OFTEN CANNOT EXCLUDE an atypical lipomatous tumour. Homogeneous fat on CT is not a licence to stop. MRI is the characterisation study; MDM2 amplification on histology is the discriminator imaging does not have.
- A superficial, small, purely fatty subcutaneous mass with thin septa and no nodules can be called a lipoma. Anything deep, large, or internally complex cannot.
The classic misread
- Calling an echogenic neck lump a lipoma on ultrasound alone.
- Not saying whether the mass is subcutaneous or intramuscular.