Answer: Liposarcoma

Histology: This lesion has nodules of small, bland appearing lymphocytes throughout, centered upon larger capillaries. Between the fat lobules of this lesion, within fibrous septa, there are scattered cells with enlarged, markedly hyperchromatic, nuclei. Within a mesenteric/retroperitoneal lesion, this is sufficient for the diagnosis of well-differentiated liposarcoma, lymphocyte rich type.

Discussion: The lesion mimics sclerosing mesenteritis, but sclerosing mesenteritis lacks the marked atypical cells seen in the current case. The lymphoid infiltrate in lymphocyte-rich liposarcoma can be dense and mimic lymphoma; however, the immunophenotype typically reveals a mixed population of B and T cells in these cases. Lipomas basically do not exist within the mesentery and retroperitoneum. Lipomas lack the markedly atypical cells seen focally in the current case.

The current case fits under the designation of well-differentiated liposarcoma of the inflammatory (lymphocyte-rich) type. Such lesions can easily be mistaken for inflammatory pseudotumors or mesenteritis unless careful attention is paid for the presence of atypical lipoblasts. Thorough examination of these lesions is useful, as typical non-inflamed areas are usually found in which the lipoblasts may be more readily apparent. The behavior of lymphocyte-rich liposarcoma appears to be similar to that of other types of well-defined liposarcoma; namely, relentless local recurrences in the retroperitoneum with potential for eventual dedifferentiation and metastasis.