Intermuscular lipomas are lipomas located deep between muscles.
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Terminology
Intermuscular lipomas are sometimes called ‘infiltrating lipomas’ since they can involve both the intramuscular and the tissue between muscles 1.
Epidemiology
Intermuscular lipomas are much less common than superficial lipomas and are found most often in middle-aged patients from the 4th to 6th decades of life, with men being more commonly affected 1-3.
Diagnosis
The diagnosis can be often made by typical imaging criteria, definite diagnosis is based on histology.
Clinical presentation
They might be found incidentally, or patients might present with a palpable nonspecific lump. They are usually larger on clinical presentation than superficial lipomas 1,2.
Pathology
Lipomas consist of mature adipocytes identical to normal adult fatty tissue 1.
Location
Intermuscular lipomas are most often found between large muscles of the following locations 1,2:
lower extremity
trunk
shoulder
upper extremity
Intermuscular lipomas of the chest wall, hand, and feet are rare as well as in the retroperitoneum.
Macroscopic appearance
Intermuscular lipomas are usually yellowish, well-circumscribed lesions surrounded by a thin capsule located between muscles 1.
Microscopic appearance
Histologically adipocytes of intermuscular lipomas are usually larger, possibly with microscopic, neighboring muscular infiltration 1,2.
Radiographic features
Plain radiograph
A large intermuscular lipoma can appear as lucency on a plain radiograph.
CT
Intermuscular lipomas usually show the following features 1:
homogeneous, hypodense soft tissue mass within the musculature
typically with Hounsfield measurements in the -60 to -120 range
frequently show thin septae
MRI
MRI usually shows a fat-containing homogeneous mass within a muscle, which is isointense to subcutaneous fat in all sequences, frequently showing subtle thin intralesional septae 1.
Signal characteristics
T1: homogeneous, high signal intensity
T2: homogeneous, high signal intensity
T2FS/IMFS: homogeneous, low signal intensity
T1C+: no enhancement, except for possible subtle enhancement of the fibrous capsule
Radiology report
The radiological report should include a description of the following 5:
form, location and tumor size
tumor margins
presence of focal nodular patchy non-fatty tumor components
presence and character of septae
relation to muscular fasciae and skeletal muscles in soft tissues
relationship to local nerves and vessels
Treatment and prognosis
Treatment options depend on tumor size, location symptoms, and overall patient condition and include wide resection, which is curative.
Differential Diagnosis
Differential diagnosis includes the following 2,5: