A retroperitoneal tumor of such large size and histology (spindled cells) has to raise the possibility of dedifferentiated liposarcoma. I urge that any "fat" attached to the tumor be submitted for microscopic examination to detect any feature for a pre-existing and residual low grade liposarcoma. Retroperitoneal liposarcomas are not rare, and are the most common large retroperitoneal tumors in adults. Most of them are low grade or well differentiated liposarcoma (rather than the myxoid or pleomorphic variants of liposarcomas). Microscopically, retroperitoneal well differentiated liposarcomas appear identical to the well circumscribed atypical lipomatous tumors (atypical lipomas) of neck, shoulder, upper back and other regions in extremities. However, retroperitoneal well-differentiated liposarcomas are slow-growing, very large, not encapsulated and very difficult to resect completely due to infiltrative borders.

A minority of retroperitoneal liposarcomas may, either at the time of initial diagnosis or at the time of local recurrence, contain nodule(s) of non-lipogenic sarcomatous elements. Most of these non-lipogenic elements appear like fibrosarcoma or MFH, and often display features of high grade sarcoma. Sometimes, the sarcomatous elements are that of a low grade fibrosarcoma (consisting exclusively of spindled cells). Very rarely, features of high grade osteosarcoma, leiomyosarcoma, chondrosarcoma, rhabdomyosarcoma and malignant peripheral nerve sheath tumor are found. These tumors are known as dedifferentiated liposarcomas. They are presumed to evolve from the pre-existing well differentiated liposarcomas. Unlike well differentiated liposarcomas, dedifferentiated liposarcomas of the retroperitoneum have definite metastatic potential (in 1~18% of cases). No matter what kinds of sarcomatous elements are found in a retroperitoneal dedifferentiated liposarcoma, its clinical behavior is not the same as high grade fibrosarcomas, leiomyosarcomas, osteosarcomas, chondrosarcomas, rhabdomyosarcomas, or MPNST. Dedifferentiated liposarcomas are not unique to the retroperitoneum. Another common location for it is the inguinal area and spermatic cord.