Answer: Dedifferentiated liposarcoma

Histology: This tumor has two components. The first component is that of a well-differentiated liposarcoma featuring markedly atypical lipoblasts within a background of differentiated fatty tumor with fibrosis. This area has features of well-differentiated lipoma-like and sclerosing liposarcoma. Adjacent to this lesion is a highly cellular, non-lipogenic spindle cell sarcoma, featuring high cellularity, pleomorphism, and mitotic activity. These tumor cells have dense eosinophilic cytoplasm. Scattered cells demonstrate tadpole and strap shapes. These features suggest skeletal muscle differentiation, which is confirmed by immunostains for desmin in concert with immunostains for myogenin. The latter is fairly specific for skeletal muscle differentiation.

Discussion: Leiomyosarcoma would not be associated with the lower grade fatty component evident in this tumor. Leiomyosarcoma should also not stain for myogenin. Similarly, malignant fibrous histiocytomas lack a lipogenic component, and should not stain for myogenin. True MFH of the retroperitoneum is probably very rare: most tumors with this appearance in the retroperitoneum are probably dedifferentiated liposarcomas. One should always consider the possibility of angiomyolipoma when confronted with a neoplasm with both adipocytic and myogenic differentiation. The current tumor demonstrates marked atypia in both the adipocytic and myogenic components, and lacks the abnormal disorganized blood vessels of angiomyolipoma. It did not label for HMB45 as angioyolipomas do.

The diagnosis of dedifferentiated liposarcoma is rendered when a non-lipogenic sarcoma develops in the context of a well-differentiated liposarcoma. The non-lipogenic sarcoma often has the appearance of malignant fibrous histiocytoma. The current case shows rhabdomyoblastic differentiation. The presence of a dedifferentiated component indicates that the lesion now has metastatic potential. The lesion will behave like a high-grade sarcoma.