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A lipoma is a benign tumour composed of mature adipocytes, and is the most common soft tissue tumour seen is approximately 2% of the population 5

Demographics and clinical presentation

Typically lipomas are superficial and present in adulthood (5th-7th decades) as a soft painless mass in the trunk or proximal extremities. These are likely to have been present for many years, and may change size with weight fluctuation.

Pathology

Simple lipomas are circumscribed encapsulated soft masses appearing entirely made 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 4.

A marked minority of lipomas are considered 'deep' (i.e. deep to muscle and or fascia), only representing 1% of all lipomas 5. Deep lipomas should be viewed with greater suspicion as a high percentage of deep fatty masses are in fact liposarcomas 5.

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 5.

Associations

In some cases multiple lipomas are associated with syndromes, including :

Specific locations

Radiographic features

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 lipomas1.

Plain film

Lipomas may be appreciated as a region of low density exerting mass effect. Calcification may be present in up to 11% of cases 5

CT

Appearances on CT are characteristic demonstrating low density (typically approximately - 65 to - 120 HU) 3-5.

MRI

MRI is the preferred modality of choice, 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 local anatomy.

As expected lipomas follow subcutaneous fat on all sequences:

  • T1
    • high signal
    • saturates on fat saturated sequences
    • no or minimal enhancement
  • T2
    • 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% specific1. Similarly if suspicious features are present then the sensitivity of MRI is 100% 1, although specificity is lower, as some masses with atypical features will nonetheless be lipomas.  

Ultrasound

Lipomas appear as soft variably echogenic masses. Although if encapsulated, the capsule is usually difficult to identify on ultrasound 5

According the recent study (2004) 6

  • 29% were hyperechoic
  • 22% were isoechoic
  • 29% were hypoechoic
  • 20% were of mixed echogenicity

Treatment and prognosis

If all characteristics are those of a simple lipoma, and no local symptoms such as pain are present then no treatment is generally required. If any concern exists then biopsy or excision is required, with care taken about the approach in case the lesion is malignant. Recurrence rates of 4-5% are reported (most in deeper lesions) 5.

Differential diagnosis

In general there is little differential for a classic lipoma. The main differential is:

  • liposarcoma
    • low grade tumours are difficult to differentiate from lipomas, and can have relatively benign clinical course but suffer from high rate of recurrence1.
  • normal adipose tissue

In certain locations then other fatty masses should be considered :

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