The term undifferentiated pleomorphic sarcoma (UPS) refers to a group a soft tissue sarcomas that cannot be otherwise classified based on histologic, immunohistochemical, or cytogenetic features. These lesions were previously categorized as malignant fibrous histiocytomas, a term which has been abandoned, as most tumors within the group have since been otherwise classified.

These tumors comprise approximately 20%-25% of all soft tissue sarcomas, are typically high grade, and are often associated with a poor prognosis. The diagnosis is made after all other defined entities have been excluded. The etiology of these aggressive soft tissue sarcomas is unknown. They occur most commonly in the skeletal muscle, usually involving the arms or legs, or the retroperitoneum, typically beneath the fascia. Occasionally, UPS is confined to the subcutis and may rarely be seen in the deep dermis.

These tumors characteristically occur later in adult life and are rare in young people. There is a predominance of cases in men over women. UPS often presents as a painless growing mass on the proximal leg. Less frequently, the proximal arm is affected, but lesions may occur on any area of the body. The retroperitoneum is another site of predilection. If the lesion grows very large, it may cause pain, numbness, or tingling. If the abdomen is involved, abdominal pain, constipation, or loss of appetite may occur. Some patients will develop fevers and weight loss. Risk factors for development of UPS include older age and history of previous radiation therapy, although most patients do not have identified risk factors.

With respect to prognosis, up to 50% of patients die from their disease within 5 years, and the presence of metastatic disease confers a median survival of 8-12 months. Most metastases occur in the lungs (90%); other sites of metastasis include the bones (8%) and liver (1%). Age over 70, a lower extremity tumor, and the presence of metastatic disease at the time of presentation were associated with lower overall survival in one large study. Factors associated with local recurrence included positive surgical margins and older age.