Last revised by Wojciech Micał on 16 Sep 2022

Lipomas are benign tumors composed of mature adipocytes. They are the most common soft tissue tumor, seen in ~2% of the population. 

Patients typically present in adulthood (5th-7th decades).

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

Diagnostic criteria according to the WHO classification of soft tissue and bone tumors (5th edition) 10:

  • essential: yellow-tan circumscribed mass; uniform proliferation of mature adipose tissue with atypical hyperchromatic stromal cells
  • desirable: absence of giant marker/ring chromosomes / MDM2 amplification

Typically lipomas are subcutaneous in location and present in adulthood 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.

The etiology of lipomas is unknown 10. In 5-15% of patients, lipomas are multiple, and ~33% of these will be familial 5.

Benign lipomas are circumscribed soft masses, usually encapsulated, and composed almost entirely of fat. Superficial lipomas are usually <5 cm whereas deep lipomas can be >20 cm 10. A small amount of non-adipose components are often present, representing fibrous septa, areas of fat necrosis, blood vessels, and interposed muscle fibers. Any non-adipose components must be carefully assessed to exclude a more aggressive component. Lipomas containing metaplastic cartilage or bone are called chondrolipomas or osteolipomas respectively 10.

Histology demonstrates mature adipocytes with no cellular atypia or pleomorphism 4.

Intramuscular lipomas have nearly identical histology to superficial lipomas; however, intramuscular lesions tend to invade the adjacent musculature and commonly lack a capsule. These are sometimes known as infiltrating lipomas. In contradistinction, intermuscular lipomas do not exhibit local invasion and tend to be lobular or dumbbell-shaped, easily separated from adjacent soft tissues during surgical resection 6,7.

Most lipomas are superficial in the subcutaneous tissues 10. A minority of lipomas are found in deeper locations and include intramuscular, intermuscular, intrathoracic, and retroperitoneal lipomas. 

Superficial lipomas are typically well-circumscribed ovoid masses with homogeneous imaging characteristics of fat. A thin capsule, very thin septations (<2 mm), and scattered small areas of soft tissue density are common; however, avidly enhancing, thick/nodular septa or evidence of local invasion are suggestive of malignancy.

Large lipomas may be appreciated as a region of low density exerting mass effect. Calcification may be present in up to 11% of cases, although more commonly associated with well-differentiated liposarcoma 5

Lipomas appear as soft variably echogenic masses, commonly encountered on ultrasound. If encapsulated, the capsule may be difficult to identify on ultrasound 5. There is a wide range of appearances of biopsy-proven lipomas, with wide inter-reader variability 8:

  • hyperechoic: 20-52%
  • isoechoic: 28-60%
  • hypoechoic: 20%

They can also display other ultrasound features 9:

  • no acoustic shadowing
  • no or minimal color Doppler flow

Heterogeneous echotexture, more than minimal color Doppler flow, or large size is suspicious for liposarcoma.

Classic appearances are of a superficial circumscribed, low attenuation mass (typically approximately -65 to -120 HU) 3-5 with minimal internal soft-tissue component. Again, areas of calcification may be present although are more frequently associated with well-differentiated liposarcoma 5.

Deeper or larger lesions may have scattered areas of internal soft-tissue density, often more apparent on CT versus MRI. These may represent areas of fat necrosis, fibrous tissue, blood vessels, or muscle fibers; these lesions cannot be confidently differentiated from liposarcoma by imaging.

Intramuscular lipomas may invade and interdigitate with the associated skeletal muscle, resulting in a characteristic striated appearance that may help distinguish them from liposarcoma 5.

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:

  • 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% specific 1. 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.

Size and location should be described. Location not only includes the anatomical site but whether the mass is superficial or deep, intramuscular, etc. When reporting lipomas on MRI, the article lipoma lipoma vs well-differentiated liposarcoma covers the differentiating factors well. 

If all characteristics are those of a simple lipoma, and no local symptoms such as pain are present, then no treatment is generally required. However, resection for cosmesis is also a reasonable indication. 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.

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

  • liposarcoma: low-grade tumors are difficult to differentiate from lipomas, and can have a relatively benign clinical course but suffer from a high rate of recurrence 1
  • normal adipose tissue

In certain locations, other fatty masses should be considered:

  • deep lipomatous masses should be assessed carefully, as they are more likely to be malignant and retroperitoneal fatty masses should be considered a well-differentiated liposarcoma until proven otherwise 5

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Cases and figures

  • Figure 1: histology - spindle cell lipoma
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  • Figure 2: gross pathology
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  • Case 1: within spinal canal (T1)
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  • Case 2: of corpus callosum (FLAIR)
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  • Case 3: deep to deltoid (PD)
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  • Case 4: within abdominal wall
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  • Case 5: right parotid lipoma
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  • Case 6: pancreatic lipoma
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  • Case 7: lipoma of chest wall
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  • Case 8
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  • Case 9: muscular (thenar region)
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  • Case 10: soft tissue lipoma of hand
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  • Case 11: on right (neck)
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  • Case 12: right parapharyngeal lipoma
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  • Case 13: intraosseous lipoma
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  • Case 14: pancreatic lipoma
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  • Case 15: pectoralis muscle lipoma
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  • Case 16: knee lipoma - ultrasound
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  • Case 17: large lipoma of the neck
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  • Case 18: submandibular
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  • Case 19: parosteal
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  • Case 20: subcutaneous lipoma in nape of neck
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  • Case 21: intradeltoid lipoma
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  • Case 22: intermuscular lipoma
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  • Case 23: lipoma of thigh
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  • Case 24: large lipoma of scalp
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  • Case 25: parotid lipoma
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  • Case 26: thigh
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  • Case 27: arm
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  • Case 28: lipoma causing posterior interosseous neuropathy
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  • Case 29: retroperitoneal lipoma
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  • Case 30: Neck intermuscular lipoma
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  • Case 31
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