<unk>  (UPS), also termed<unk>  myofibrosarcoma, high-grade myofibroblastic<unk> , and high-grade myofibrosarcoma, is characterized by the World Health Organization (WHO), 2020, as a rare, poorly differentiated neoplasm, i.e. an abnormal growth of cells that have an unclear identity and/or cell of origin. WHO classified it as one of the<unk> /unclassified<unk> s in the category of tumors of uncertain differentiation.<unk> s are cancers known or thought to derive from mesenchymal stem cells that typically develop in bone, muscle, fat, blood vessels, lymphatic vessels, tendons, and ligaments. More than 70<unk>  subtypes have been described. The UPS subtype of these<unk> s consists of tumor cells that are poorly differentiated and may appear as spindle-shaped cells, histiocytes, and giant cells. UPS is considered a diagnosis that defies formal sub-classification after thorough histologic, immunohistochemical, and ultrastructural examinations fail to identify the type of cells involved. The diagnosis of UPS initially included the malignant fibrous histiocytomas (MFH). MFH are now regarded as a wastebasket category of various<unk>  types including<unk> -like carcinomas and melanomas. Studies strongly suggest that MFH tumors are not derived from histiocytes (cells descended from blood monocytes), but rather from mesenchymal cells. UPS had also been regarded as a more aggressive and metastasizing form of the low-grade myofibroblastic<unk> s and intermediate‐grade myofibroblasic<unk> s. WHO has combined low- and intermediate-grade myofibroblastic<unk> s into a single entity, low-grade myofibroblastic<unk> s, and categorized it one type of the intermediate (rarely metastasizing) fibroblastic and myofibroblastic tumors quite distinct from UPS. Because of their low incidence and frequent grouping with what are now considered to be other<unk>  types, past findings on the clinical behaviour, proper treatment, and prognosis of UPS may be revised with further study. The majority of UPS tumors are highly aggressive, often recur after surgical removal, and often metastasize. They are treated with a combination of surgical resection, radiotherapy, and/or chemotherapy. More recently, UPS tumors have been treated with antibody therapy, i.e. antibodies which in the case of UPS bind to specific antigens on the surface of T-cells (a type of lymphocyte) and thereby promote the ability of these T-cells to organizes an attack on UPS tumor cells.
Undifferentiated pleomorphic sarcoma
Low-grade fibromyxoid sarcoma
Solitary fibrous tumor
Sclerosing epithelioid fibrosarcoma
Myxoid liposarcoma
Angiomatoid fibrous histiocytoma
Myxoid tumor
Epithelioid sarcoma