"to produce flesh, grow fleshy", from sarx (genitive sarkos) "flesh", + -oma. Forms of liposarcomas Liposarcomas are generally large tumors (>10 cm) but can be of almost any size. They occur mainly in adults with only 0.7% of cases occurring in those <16 years old. In adults, liposarcomas occur predominantly in and after middle-age. The very rare cases occurring in children and adolescents are diagnosed predominantly as being the myxoid liposarcoma form. The five liposarcoma forms must be distinguished not only from each other but also from certain other soft tissue tumors. These other tumors along with some of their distinguishing histopathologic features are: 1) dysplastic lipomas (i.e. benign humors that have sites of tissue necrosis and neoplastic, variably-sized fat cells containing variable sized/shaped nuclei; these neoplastic cells, unlike most neoplastic cell in the liposarcomas, do not overexpress the MDM2 gene); 2) atypical spindle cell lipomas (i.e. benign tumors with mildly atypical spindle-shaped cells in a fibrous-to-myxoid stroma intermixed with vacuolated lipoblasts and variable-sized adipocytes with atypical nuclei; 3) pleomorphic lipomas (i.e. benign tumors characterized by giant cells with overlapping nuclei); and 4) solitary fibrous tumors (i.e. tumors, up to 22% of which exhibit malignant behavior, consisting of spindle- or ovoid-shape cells within a collagenous background stroma intermixed with blood vessels with a characteristic staghorn shape). Atypical lipomatous tumor/well-differentiated liposarcoma Together, atypical lipomatous tumors (ALTs) and well-differentiated liposarcomas (WDLs) account for 4045% of all liposarcomas. They rarely if ever metastasize and therefore are regarded as benign or premalignant tumors. However, they are locally invasive and may transform to a more aggressive and potentially metastasizing liposarcoma, i.e. a dedifferentiated liposarcoma. Furthermore, a surgically removed atypical lipomatous tumor/well-differentiated liposarcoma may recur as a dedifferentiated liposarcoma. Presentation ALTs and WDLs are considered virtually identical tumors except that by definition ALTs designate tumors that develop in the arms or legs while WDLs designate tumors that develop in less surgically accessible sites such as the deep, centrally-located soft tissues of the retroperitoneum, paratesticular region (i.e. area within the scrotum including the testes, spermatic cord, testicular tunic, epididymis, and appendix of testis), oral cavity, and eye socket. This terminology has prognostic implications: less than 7% of ALT tumors convert to dedifferentiated liposarcomas within a median time of 7 years while 17% of WDL tumors convert to this more malignant liposarcoma within a median time of 8 years. ALT and WDL (hereafter termed ALT/WDL) tumors typically present