Undifferentiated pleomorphic sarcoma is a common form of soft tissue malignancy in which the tissue type of origin is not discernable. It is therefore a diagnosis of exclusion.
Pleomorphic sarcomas represent the largest group of soft tissue sarcomas in older adults, slightly more frequently in men. The incidence is about 1/100,000 per year.
Pleomorphic sarcoma typically occurs in the limbs and only rarely in the trunk. Pleomorphic sarcoma arising as the dedifferentiated part of a liposarcoma is a notable exeption and often occurs in the retroperitoneum.
Aetiology and Pathogenesis
No specific causative factors are known. About 2-3% arise in radiated fields.
Lesions grow rapidly but are uncommonly painful. Metastasis is infrequent at diagnosis (5%) but often occurs after treatment is completed.
The tumour is often large (5-10 cm +) and locally invasive. It may appear encapsulated. The cut surface is white to tan and has a fibrous appearance. Macroscopic areas of necrosis and haemorrhage are often visible.
Tumours vary in appearance. A common feature is significant anaplasia, with pleomorphism, mitoses, giant cells and hyperchromatism noted. Foamy macrophages (histiocytes) are commonly present.
Immunohistochemistry is used to divide pleomorphic sarcoma into those arising from a particular tissue (eg. pleomorphic liposarcoma, pleomorphic rhabdomyosarcoma) and those in which tissue origin can not be discerned. This last group is defined as undifferentiated pleomorphic sarcoma.
The genetic changes in undifferentiated pleomorphic sarcoma are, like other pleomorphic tumours, complex and no distinct translocation or amplification has been determined.