or at the site were an ALT/WPL tumor was surgically removed. Individuals with a de novo diagnosis of this tumor may have had an ALT/WDL that progressed to a dedifferentiated liposarcoma but went undetected because it developed asymptomatically in a highly sequestered site such as the retroperitoneum or abdominal cavity. Many of the dedifferentiated liposarcoma tumors' clinical and genetic features are similar to those found in ALT/WDL tumors. Presentation Dedifferentiated lipoosarcomas (DDL) occur most frequently in middle-aged and older adults with a peak incidence in their sixth to eighth decades. Rarely, these tumors have developed in children and adolescents. DDL tumors most commonly occur in the retroperitoneal space but, similar to ALT/WDL, may occur in the extremities, paratesticular area, mediastinum, head, or neck. Less than 1% of all DDLs develop in superficial soft tissues or the eye socket. At presentation, DDL tumors typically are painless, large, may have been slowly and progressively enlarging for years, and on routine X-rays contain areas of calcium deposition (exemplified by Fig. 1 in the Histopathology of liposarcomas section). Less commonly, affected individuals have signs and/or symptoms due to their tumor's impingement on an organ (e.g. abdominal pain caused by blockage of the intestines or urinary tract obstruction caused by blockage of the urethra). Very rarely, individuals with DDL present with one or more signs or symptoms of chronic inflammation (see B symptoms) and/or one of the endrocrine, neurological, mucocutaneous, hematological, or other tissue-related paraneoplastic syndromes. The signs and symptoms of chronic inflammation and the various paraneoplastic syndromes are caused by the tumors' secretion of cytokines, hormones, prostaglandins, and/or other systemically acting agents; they completely disappear after the DDL is successfully treated. Pathology The histopathological appearance of DDL tumors (see Fig. 2 in the below Histopathology of liposarcomas section) varies widely but most frequently exhibits features of undifferentiated pleomorphic sarcomas (which are tumors densely populated with variably sized and shaped cells containing variability sized and shaped nuclei) or spindle cell sarcomas (which are tumors consisting of spindle-shaped cells in a connective tissue background). Different parts of DDL tumors often show variations in the appearances of their background connective tissues: these tissues may be myxoid (i.e. consisting of a clear, mucus-like substance which when stained using a standard H&E stain method appears more blue or purple than the red color of normal tissues) or myxocollagenous (i.e. high collagen fiber content in a myxoid background), and, in