Answer: Sclerosing well-differentiated liposarcoma

Histology: The specimen is from a fatty tumor which has areas composed of relatively normal appearing fatty tissue admixed with sclerosing dense areas of fibrosis. There are atypical cells with hyperchromatic nuclei and occasional lipoblasts can be identified within the more lipomatous portions of the tumor. In addition, scattered large atypical cells often with multiple nuclei are found in the more fibrotic bands running through the tumor.

Discussion: Liposarcomas may occur in various regions but the extremities, particularly the thigh, and the retroperitoneum are the most common sites for malignant adipose tumors. The retroperitoneum is the second most common location for liposarcomas. In this site, the tumors are usually quite large in size, as in the case presented here. Liposarcomas in the retroperitoneum often take a while to be recognized and by the time the patient comes to operation they are usually of a large size. In general, liposarcomas are divided into multiple subtypes including well-differentiated liposarcoma, myxoid liposarcoma, round-cell liposarcoma, and pleomorphic liposarcoma. This particular tumor falls into the category of a well differentiated liposarcoma of the sclerosing type. In order to establish the diagnosis one must identify lipoblasts. These cells characteristically have lipid-like droplets that are defined and the nucleus is usually distorted by the pressure of the expanding lipid within the cell. These cells may vary in size and some may actually reach giant cell proportions. This tumor can be distinguished from the myxoid variant of the well differentiated liposarcoma because it lacks myxoid areas as well as the classical vascular proliferation associated with the myxoid liposarcoma. It is impostant to distinguish this well differentiated liposarcoma from a lipoma. The scattered lipoblasts as well as the hyperchromatic atypical nuclei help in making this distinction. These cells are most commonly found in the fibrosing area of the tumor. This sclerosing type of well differentiated liposarcoma most often does occur in the retroperitoneum or inguinal regions. It may also be found in other sites including extremities. Liposarcomas of the retroperitoneum have a relatively high rate of recurrence, probably due to the difficulty in obtaining a complete excision of the tumor mass. Although multiple recurrences with this type of tumor are reported, patients with the low-grade well differentiated types of liposarcoma survive for multiple years despite local recurrences. Metastases of liposarcomas correlate with the histologic type and in the well differentiated liposarcomas metastases are uncommon.