Intramuscular lipoma

Last revised by Dr Rania Adel Anan on 07 Aug 2022

Intramuscular lipomas are deep-seated lipomas located within a muscle.

Intramuscular lipomas share the term ‘infiltrating lipoma’ with intermuscular lipomas.

Intramuscular lipomas account for about 1% of all lipomas and occur in all age groups with the most occurring between the 5th to 7th decades of life. There seems to be a female predominance 1.

Patients might present with unspecific swelling of the tumor or be found incidentally. Rare presentations are pain or paresthesia due to nerve entrapment 1,2.

Intramuscular lipomas consist of mature adipocytes identical to normal adult fatty tissue 1, the exact etiology remains unclear.

  • infiltrative
  • well defined or circumscribed

They can occur at any anatomical site but are believed to occur mainly in the large muscles of the limbs and the trunk, an exact anatomical distribution, however, has not been established. Intramuscular lipomas are rare in the hand and feet and very rare in the oral cavity 1,3.

Intramuscular lipomas are usually yellowish and located within muscle. They may show a capsule at the margin can show interdigitations and muscle fibers passing through 1.

Mature adipocytes irregularly infiltrate and replace muscle fibers and bundles in the infiltrative type 1. There is no fatty infiltration of adjacent muscle typically observed in the well-defined or well-circumscribed type.

An intramuscular lipoma can appear as a lucency, especially if large and can show radioopaque streaks.

Appears as a hyperechoic, well-defined mass with fine internal echoes or striated appearance due to interdigitations of muscular tissue.

Intramuscular lipomas usually show the following features 1:

  • hypodense soft tissue mass within the musculature
  • typically with Hounsfield measurements in the negative range (fat density)
  • can show striated appearance
  • can show thick intramuscular septae and interdigitations
  • usually, oval or fusiform shape but can vary

Usually shows either a fat-containing mass within a muscle, which is isointense to subcutaneous fat in all sequences sometimes with septae. Septae should not enhance avidly however or show nodules 2. Interlocked or intermingled muscular tissue and fibers can be observed in the infiltrative type 1. If a capsule is present and found outside the intramuscular lipoma, there should not be muscular fibers in the main mass 1.

Treatment depends on tumor size location and symptoms and includes watchful waiting or wide resection.

  • well-differentiated liposarcoma (dumb-bell shaped, should be suspected if septae markedly enhance)

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Cases and figures

  • Case 1: brachialis
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  • Case 2: latissimus dorsi
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  • Case 3: latissimus dorsi
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  • Case 4: deltoid (heart-shaped)
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  • Case 5: Latissimus dorsi
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