Atypical spindle cell/pleomorphic lipomatous tumors or atypical spindle cell lipoma are benign adipocytic soft tissue neoplasms with a variable proportion of atypical spindle cells, pleomorphic cells adipocytes and other cells with no risk for dedifferentiation but a low risk of local recurrence. This entity has been included in the WHO classification of soft tissue tumors in 2020.
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Diagnosis
Diagnosing atypical spindle cell/pleomorphic lipomatous tumors is based on typical histological and molecular pathological features.
Diagnostic criteria
Diagnostic criteria according to the WHO classification of soft tissue and bone tumors (5th edition) 1:
- variable amounts of atypical spindle cells, adipocytes uni or multivacuolated lipoblasts and pleomorphic or multinucleated cells
- myxoid to collagenous extracellular matrix
The following criteria are desirable:
- RB1 deletion
- lack of MDM2 or CDK4 amplification
Epidemiology
Atypical spindle cell/pleomorphic lipomatous tumors are mostly seen in patients above 30 years with a peak incidence in the sixth decade but can generally occur in a wide age range. Men are slightly more frequently affected than women 1,2.
Clinical presentation
The typical presentation is a slow-growing nodule or mass possibly associated with tenderness 1.
Pathology
Atypical spindle cell/pleomorphic lipomatous tumors are characterized by a variable amount of atypical spindle cells adipocytes, lipoblasts and pleomorphic or multinucleated cells found in a fibrous and/or myxoid matrix 1-3.
Etiology
The etiology of atypical spindle cell/pleomorphic lipomatous tumors is unknown 1.
Location
Atypical spindle cell/pleomorphic lipomatous tumors are usually found in the subcutaneous and deep soft tissues of the limbs and limb-girdle areas and less frequently in the head and neck area, the genital region or the back and trunk. Only occasionally they are seen in visceral locations 1-3.
Macroscopic appearance
Macroscopically atypical spindle cell/pleomorphic lipomatous tumors are characterized by a nodular or multinodular growth pattern and are unencapsulated with ill-defined tumor margins 1.
Microscopic appearance
Histologically atypical spindle cell/pleomorphic lipomatous tumors feature a wide range of microscopic appearances mostly dependent on the relative amount of adipocytes, lipoblasts, atypical spindle cells and multinucleated pleomorphic cells as well as a variable myxoid and/or collagenous background. Some features include the following 1-3:
- predominantly mature adipocytes of variable shape and size and mild too moderate atypia
- variably small vacuolated or multivacuolated lipoblasts
- intermixed bizarre hyperchromatic and/or pleomorphic multinucleated cells
- no necrosis
- scarce mitotic figures
- broad spectrum from paucicellular with abundant extracellular matrix to a cellular appearance with a lesser extracellular matrix component
- ill-defined fibrous pseudocapsule
Immunophenotype
Immunohistochemistry stains can be variably positive for CD34, S100 and desmin and can show a loss of nuclear RB expression in the majority of cases. There is an absence of coexpression of MDM2 and CDK4, however, there can be weak expressions of either one of them 1-5.
Genetics
Atypical spindle cell/pleomorphic lipomatous tumors are associated with deletions or losses on the 13q14 chromosome in particular the RB1 and its flanking genes 1-5.
Radiographic features
Description of radiological features of this entity is very scarce in the literature so far. Presumably, the tumors will have an unspecific lobular or multilobular appearance with a possibly visible ill-defined fibrous capsule.
MRI
MRI is probably the most suitable imaging modality for the characterization of these tumors. Due to its variable amount of different cells, the appearance is expected to be also variable and quite heterogeneous.
Radiology report
The radiological report should include a description of the following:
- form, location and size
- tumor margins and transition zone
- relations to the muscular fascia
- relation to neurovascular structures
Treatment and prognosis
Atypical spindle cell/pleomorphic lipomatous tumors are benign lesions and have an excellent prognosis if they are completely excised. The local recurrence rate is about 10-15% 4. There is no known risk of metastases 1-3.
History and etymology
The neoplasms were first described under the term ‘spindle cell liposarcoma’ by the Italian pathologist and oncologist Angelo Paolo Dei Tos in 1994 5,6. The terms ‘spindle cell liposarcoma’ and ‘fibrosarcoma-like lipomatous neoplasm’ are now discouraged 1-3.
Differential diagnosis
Tumors or conditions which can mimic the presentation and/or appearance of atypical spindle cell/pleomorphic lipomatous tumors include 2.