Well-Differentiated Liposarcoma With Hibernoma-Like Morphology.

Well-differentiated liposarcoma (WDL) can show a morphologic spectrum, including lipoma-like, sclerosing and inflammatory subtypes. It does not metastasize but can dedifferentiate, acquiring metastatic potential. Hibernomas are benign neoplasms that show variable differentiation toward brown fat, and can sometimes occur in the abdomen or retroperitoneum. We illustrate a case of retroperitoneal WDL that showed extensive hibernoma-like morphology, with sheets of multivacuolated adipocytes of varying sizes, with abundant cytoplasm and numerous lipid vacuoles or granular eosinophilic cytoplasm. However, very focally there were fibrous septa containing spindle cells with enlarged, hyperchromatic, mildly pleomorphic nuclei. Further sampling showed areas of typical WDL, with lobules of mature fat intersected by fibrous septa containing atypical, enlarged spindle cells, as well as small foci of dedifferentiation. Immunohistochemistry for CDK4 and p16 showed strong and diffuse nuclear expression in the hibernoma-like areas, and fluorescence in situ hybridization showed MDM2 gene amplification, all in keeping with WDL. We highlight hibernoma-like morphology as part of the histologic spectrum of WDL, and recognition of this variant is important for correct treatment and prognostication.