Case Id: JSSMCRADC0055
Lipomas are benign tumours composed of mature adipocytes. They are the most common soft tissue tumour seen in approximately 2% of the population.
Patients typically present in adulthood (5th-7th decades).
Typically lipomas are superficial and present as a soft painless mass in the trunk or proximal extremities. They are likely to have been present for many years, and may change size with weight fluctuation.
Simple lipomas are circumscribed encapsulated soft masses, made nearly entirely of fat. Occasionally solid components will be present (blood vessels, muscle fibres, fibrous septae, and fat necrosis), which need to be carefully assessed to ensure that these do not represent a more aggressive component. Histology demonstrates mature adipocytes with no cellular atypia or pleomorphism.
A marked minority of lipomas are considered 'deep' (i.e. deep to muscle and or fascia), only representing 1% of all lipomas. Deep lipomas should be viewed with greater suspicion as a high percentage of deep fatty masses are in fact liposarcomas.
Occasionally lipomas will be infiltrating, spreading through a muscle (also known as intramuscular lipomas).
In 5-15% of patients lipomas are multiple, and approximately a third of these will be familial
Lipomas are typically well circumscribed ovoid masses with homogeneous imaging characteristics of fat. A thin capsule and very thin septations (<2 mm) are often seen. Presence of a non-fatty soft tissue component, thick or nodular septae or evidence of invasion are suggestive of malignant transformation, although blood vessels, muscle fibres, fibrous septae and fat necrosis can be seen in lipomas.
Lipomas may be appreciated as a region of low density exerting mass effect.
Calcification may be present in up to 11% of cases.
Appearances on CT are characteristic, demonstrating low attenuation (typically approximately -65 to -120 HU) .
MRI is the modality of choice for imaging lipomas, not only to confirm the diagnosis, which is usually strongly suggested by ultrasound and CT, but also to better assess for atypical features suggesting liposarcoma. Additionally, MRI is better able to demonstrate the surrounding anatomy.
As expected, lipomas follow subcutaneous fat signal on all sequences:
- high signal
- saturates on fat saturated sequences
- no or minimal enhancement
- high signal on FSE T2
- saturates on fat saturated sequences: persistent areas of high T2 signal are worrisome
When no suspicious features are present, the diagnosis of lipoma can be made with confidence with MRI being 100% specific . Similarly, if suspicious features are present, then the sensitivity of MRI is 100%, although specificity is lower, as some masses with atypical features will nonetheless be lipomas.
Lipomas appear as soft variably echogenic masses, commonly encountered on ultrasound. If encapsulated, the capsule may be difficult to identify on ultrasound .
According a recent study, there is a wide range of appearance of biospy-proven lipomas, with wide inter-reader variability (2004) :
- hyperechoic: 20-52%
- isoechoic: 28-60%
- hypoechoic: 20%
They also tend to display other ultrasound features, such as :
- no acoustic shadowing
- no or minimal colour Doppler flow
Heterogenous echotexture, more than minimal colour Doppler flow, or large size is suspicious for liposarcoma.
In general there is little differential for a classic lipoma. The main differential is:
- low grade tumours are difficult to differentiate from lipomas, and can have relatively benign clinical course but suffer from high rate of recurrence.
- normal adipose tissue