This large fleshy mass arose in the retroperitoneum and is an example of a sarcoma. Sarcomas arise within mesenchymal tissues. This one happened to be a high-grade undifferentiated pleomorphic sarcoma (HGUPS), a large term replacing the prior term "malignant fibrous histiocytoma" which is a wastebasket term for sarcomas that do not resemble recognizable mesenchymal cells such as striated muscle (rhabdomyosarcoma), smooth muscle (leiomyosarcoma), fat (liposarcoma), blood vessels (angiosarcoma), bone (osteosarcoma), or cartilage (chondrosarcoma). The HGUPS is likely derived from mesenchymal stem cells. Regardless, sarcomas tend to be big and bad.