Myolipoma of soft tissue or extrauterine lipoleiomyoma is a benign adipocytic soft tissue neoplasm mainly consisting of well-differentiated smooth muscle cells and mature adipose tissue.
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Epidemiology
Myolipomas are rare, they are more common in women and seen in the 5th and 6th decades of life 1-3.
Diagnosis
The diagnosis of myolipoma of soft tissue is based on the location and typical pathological features.
Diagnostic criteria
Diagnostic criteria according to the classification of soft tissue and bone tumors (5th edition) 2:
combination of well-differentiated smooth muscle and adipocytes
extrauterine location
Clinical presentation
Patients might present with a palpable lump if the myolipoma is located in the trunk or extremities. Tumors in the abdominal cavity and retroperitoneum are mostly asymptomatic and found incidentally 1.
Pathology
Myolipomas consist of mature fat cells and well-differentiated spindled, smooth muscle cells. They can grow quite large if previously undetected 1,2.
Etiology
Up to the time of writing the etiology of soft tissue myolipoma is unknown 2.
Location
Myolipoma of soft tissue has been found in the following sites 2-5:
abdominal wall
pelvic cavity e.g. at a presacral location
subcutaneous tissues of the trunk and/or extremities (rarely)
Macroscopic appearance
Myolipomas are usually circumscribed and surrounded by a capsule and consist of adipose tissue and smooth muscle cells 1,2. Deep-seated extrauterine myolipomas can have a considerable size of up to 10-25 cm 2.
Microscopic appearance
Histologically myolipoma of soft tissue is characterized by the following features 2:
short fascicles of smooth muscle interspersed with mature adipose tissue
variable inflammatory and fibrous component
no necrosis
no mitotic figures
Immunophenotype
Immunohistochemistry stains are usually positive for smooth muscle actin, desmin and or caldesmon 1-3. More than half of the cases show nuclear reactivity to HMGA2 2,3.
Radiographic features
CT
Relatively circumscribed, fat-containing heterogeneous mass with hypodense fat tissue components and soft tissue density muscular components 1.
MRI
Usually shows a fat-containing heterogenous mass with interposed soft tissue density reflecting the smooth muscle components.
T1: heterogeneous intermediate signal intensity
T2: heterogeneous intermediate to high signal intensity
T2FS: heterogenous
T1C+ (Gd): heterogeneous enhancement
Radiology report
The radiological report should include a description of the following:
form, location and size
tumor margins
amount of non-adipose tissue
distance from the muscular fascia
relationship to local nerves and vessels
relationship to other organs
Treatment and prognosis
Treatment options include resection, which is curative.
History and etymology
Myolipoma of soft tissue has been first described by the American pathologists Jeanne Marie Meis and Franz Michael Enzinger in 1991 3-6.
Differential Diagnosis
leiomyoma with fatty degeneration
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