This large retroperitoneal soft tissue tumor has the histological features of that of liposarcoma. The lesion consists of areas of well-differentiated liposarcoma and areas of poorly differentiated liposarcoma. As in Figure 1 and Figure 2, the well-differentiated areas consist of predominantly of mature fat with a variable number of spindled cells with hyperchromatic nuclei and multivacuolated lipoblasts. Dense fibrotic areas containing collagen and atypical cells are noted to intersecting the fat. Dedifferentiated areas include spindled cells arranged in whorled structures as seen in Figure 3. The interface between the well-differentiated and dedifferentiated areas is typically abrupt (Figure 4). Poorly differentiated areas are shown in Figure 5 to Figure 8. The tumor is composed of lipoblasts suspended individually in a myxoid matrix with delicate plexiform capillary vascular network in Figure 5. As myxoid liposarcoma lose their differentiation, they acquire features of round cell liposarcoma, which is characterized by sheets of primitive round cells with a high nuclear/ cytoplasmic ratio and a prominent nucleolus (Figure 6). Focal cartilaginous and osseous differentiations are also identified in the poorly differentiated areas as in Figure 7 and Figure 8.
The tumor cells are negative for S100, smooth muscle actin, AMAHHF35, inhibin, pancytokeratin. They are focally positive for desmin and strongly positive for vimentin.