Answer: Myxoid liposarcoma

Histology: Within the pancreas, which surprisingly shows no panIN lesion, there is a myxoid spindle cell proliferation. The lesion has a regular branching chicken wire capillary vasculature, and it features bland plump spindled cells. At the periphery, one can see overt lipoblast formation. In other areas, the myxoid substance pools to yield a “pulmonary edema” pattern. These are the typical features of myxoid liposarcoma. This patient had a history of a myxoid liposarcoma involving the periprostatic area, so the current lesion is an example of an unusual site of soft tissue metastasis of myxoid liposarcoma. Metastasis to other wise unusual soft tissue sites is in fact characteristic of this neoplasm.

Discussion: Lipoblastoma is a typically a lesion seen in infants and young children, not adult males. Sarcomatoid carcinoma typical features a pleomorphic spindle cell proliferation, and should have an associated carcinomatous component. Nodular fasciitis would have more extravasated blood cells and would lack the regular capillary vasculature and lipoblasts of the current lesion.

Myxoid liposarcoma frequently features a t(12;16)(q13;p11) chromosome translocation, resulting in a fusion of the TLS and CHOP genes. CHOP is a transcription factor that normally negatively regulates adipocyte differentiation. TLS is an RNA binding protein that is homologous to EWS.