Myxoid pleomorphic liposarcomas are extremely rare aggressive malignant neoplasms arising from adipocytic tissues characterized by a mixture of pleomorphic and myxoid features and the absence of genetic fusions and amplifications seen in well-differentiated liposarcoma and other lines of differentiation.
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Epidemiology
Myxoid pleomorphic sarcomas are extremely rare and affect children and young adults up to the age of 30 years on most occasions. Women are more frequently afflicted 1-3.
Associations
There have been associations between myxoid pleomorphic liposarcoma and Li Fraumeni syndrome 4-6.
Diagnosis
The diagnosis of myxoid pleomorphic liposarcoma is established histologically and in selected cases by molecular pathology.
Diagnostic criteria
Diagnostic criteria according to WHO classification of soft tissue and bone tumors (5th edition) 1-3
- combination of the following:
- bland areas looking like conventional myxoid liposarcoma
- more cellular areas resembling pleomorphic liposarcoma
The following criterion is desirable in selected cases:
- absence of FUS/ESWR1-DDT3 gene fusions and amplifications of the MDM2 gene 1
Clinical presentation
The most common presentation is that of a large soft tissue mass 1.
Pathology
Myxoid pleomorphic liposarcomas are highly malignant neoplasms characterized by an admixture of myxoid liposarcoma-like zones and densely pleomorphic cellular areas with pleomorphic lipoblasts as seen in pleomorphic liposarcoma 1,2.
Location
The most frequent site of involvement by myxoid pleomorphic liposarcoma is the mediastinum 1-3. There are also reports of the thigh, head and neck, perineum abdomen and back 1.
Macroscopic appearance
Myxoid pleomorphic liposarcomas are not encapsulated and are characterized by ill-defined margins 1.
Microscopic appearance
The microscopic appearance of myxoid pleomorphic liposarcomas is characterized by a mixture of myxoid and pleomorphic features and includes the following 1-3:
- infiltrative margins
- abundant myxoid matrix with plexiform vascular patterns
- scattered lipoblasts
- pleomorphic spindle cells
- pleomorphic lipoblasts
- increased mitotic activity and atypical mitosis
- occasional necrosis
Immunophenotype
Myxoid pleomorphic liposarcomas do not feature specific immunotypes 1.
Genetics
Genetically there are no FUS/ESWR1-DDIT3 fusions like in myxoid liposarcoma and no amplification involving the MDM2 nuclear gene as seen in well-differentiated or dedifferentiated liposarcoma 1-3. However, they apparently show complex chromosomal alterations 8.
Radiographic features
Descriptions about imaging characteristics dedicated to myxoid pleomorphic liposarcomas are scarce. They probably look like heterogeneous soft tissue masses with myxoid components which are characterized by high water content.
Radiology report
The radiological report should include a description of the following:
- form, location and size
- tumor margins
- amount of solid and fatty components
- relation to other organs structures
Treatment and prognosis
Myxoid pleomorphic liposarcomas are characterized by a high local recurrence, distant metastases and a poor survival rate 1.
History and etymology
Myxoid pleomorphic liposarcoma has been added as a separate entity into the WHO classification of soft tissue tumors just recently in 2020 1,2. The entity is relatively new and has been defined by the Italian pathologist Rita Alaggio and her colleagues in 2009 1-3.
Differential diagnosis
Conditions that can mimic the clinical or radiological presentation of myxoid pleomorphic sarcoma are any fast-growing soft tissue tumors and include 1:
- myxoid liposarcoma
- pleomorphic liposarcoma
- dedifferentiated liposarcoma
- myxofibrosarcoma
- fibrosarcoma
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