Undifferentiated pleomorphic sarcoma (UPS), previously known as malignant fibrous histiocytoma (MFH), is considered the most common type of soft tissue sarcoma. It has an aggressive biological behaviour and a poor prognosis.
In the majority of cases, undifferentiated pleomorphic sarcoma affects the extremities, and this article focuses on musculoskeletal involvement. For other rarer sites, please refer to specific articles:
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Terminology
Malignant fibrous histiocytoma (MFH) was removed from the 2013 WHO classification of tumours of soft tissue and bone 7.
Epidemiology
Typically, undifferentiated pleomorphic sarcomas occur in adults (range 32-80; mean 59 years) with a slight male predilection (M:F 1.2:1) 3.
Risk factors
They are the most frequent soft tissue sarcoma to occur as a result of radiotherapy 3 and are also seen on a background of Paget disease. Secondary transformation into malignant sarcomas (including undifferentiated pleomorphic sarcoma) has also been reported in fibrous dysplasia, giant cell tumour, enchondroma, chronic osteomyelitis, and osteonecrosis 8.
Clinical presentation
The presentation is usually with a painless, enlarging palpable mass.
Pathology
Undifferentiated pleomorphic sarcomas are aggressive tumours that account for 25-40% of all adult soft tissue sarcomas, making them the most common type 1. However, the classification system is becoming more restrictive, with many tumours being reclassified as variants of myogenic sarcomas 3. Currently, it accounts for ~5% of adult sarcomas 8,9.
Location
They are usually confined to the soft tissues but occasionally may arise in or from bone (1-5%) 3. Although undifferentiated pleomorphic sarcomas can occur almost anywhere in the body, they have a predilection for extremities 2,6:
lower extremities: ~50%
upper extremities: ~20%
retroperitoneum: ~16%
peritoneum: ~5-10%
abdominal viscera: extremely rare
Macroscopic appearance
Macroscopically, these tumours are typically large (5-20 cm) well-circumscribed but unencapsulated masses with a grey firm heterogeneous cut surface sometimes with areas of necrosis 2,3.
Microscopic appearance
Microscopically they are heterogeneous fibroblastic tumours made up of poorly differentiated fibroblasts, myofibroblasts, and histiocyte-like cells with significant cellular pleomorphism, storiform architecture, and also bizarre multi-nucleated giant cells 2,3. They are sometimes difficult to distinguish from other high-grade sarcomas. A number of histological subtypes have been described including 2,3:
storiform-pleomorphic: most common 50-60%
myxoid: 25% (myxofibrosarcoma)
inflammatory: 5-10%
giant cell: 5-10%
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angiomatoid
relatively non-aggressive
metastases uncommon
usually occurs in young adults/adolescents
Radiographic features
Plain radiograph
Plain radiographs will demonstrate a soft tissue mass and if arising from bone, then appear as an aggressive destructive bony lesion. In some cases, curvilinear or punctate regions of calcification may be demonstrated.
CT
The density of undifferentiated pleomorphic sarcoma is typically similar to adjacent muscle, with heterogeneous lower density areas if haemorrhage, necrosis and/or myxoid material is abundant. The soft tissue component enhances. In up to 15-20% of cases, some mineralisation is present 3,4.
MRI
MRI is the modality of choice for assessing soft tissue sarcomas, as it is best able to locally stage a tumour.
These tumours are typically relatively well-circumscribed, located within or adjacent to muscle, exerting a positive mass effect on surrounding structures due to their (usual) large size at presentation.
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T1
intermediate (to low) signal intensity, similar to adjacent muscle 3,4
heterogeneity if haemorrhage, calcification, necrosis, myxoid material present
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T1 C+ (Gd)
prominent enhancement of solid components
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T2
intermediate to high signal intensity
heterogeneity if haemorrhage, calcification, necrosis, myxoid material present
Treatment and prognosis
Most undifferentiated pleomorphic sarcomas are high grade (3 and 4) and are aggressive in their biological behaviour. They frequently metastasise (30-50% at diagnosis 2) and locally recur despite aggressive treatment. The overall 5-year survival is between 25-70% 1,4.
Prognostic factors include:
tumour size: smaller being better
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location
superficial is better
distal is better
histological grade
histological subtype: the myxoid subtype has a better prognosis compared to the storiform-pleomorphic subtype 8,9
Examples
A tumour located superficially in the subcutaneous tissues of the distal extremity, and measuring less than 5 cm, has a 5-year survival of 80%, whereas a proximal large (>5 cm) and deep tumour has a 5-year survival of 55% 5.
Treatment usually consists of aggressive en bloc resection with a wide margin. Supplementary neoadjuvant chemotherapy and radiotherapy 4 is especially useful in reducing the local recurrence rate 5. Limb-sparing surgery is usually possible 5.
Differential diagnosis
General imaging differential considerations include:
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other sarcomas and soft tissue tumours 1
benign fibrous tumours
soft tissue metastases
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