Answer: C

Histologic Description: This is a fatty lesion that has a variety of histologic appearances. Toward the edge of the lesion, the lesion is more cellular, and primitive fat cells are interspersed with a branching capillary vasculature that suggests developing fat or myxoid liposarcoma. Adjacent to these areas are myxoid pools, which again suggests myxoid liposarcoma. Toward the central part of the lesion are more differentiated fat cells that closely resemble normal fat. This neoplasm demonstrated rearrangement of the PLAG1 gene and an absence of rearrangement of the DDIT3 gene, supporting the diagnosis of lipoblastoma. On clinical grounds, the patient’s age also strongly supports lipoblastoma.

Differential Diagnosis: Well-differentiated liposarcoma typically occurs in adults, and features highly pleomorphic lipoblasts that are centered on fibrous septa in a background of differentiated mature-appearing fat. Myxoid liposarcoma would lack the zonation of the current lesion, typically occurs in older patients, and demonstrates the characteristic FUS-DDIT3 gene fusion that is not seen in lipoblastoma. Fat necrosis typically does not demonstrate the myxoid appearance seen in the current case, and has prominent histiocytes.

90% of cases of lipoblastoma occur in patients under the age of 5. Approximately 1/6th of patients have an associated disorder of the central nervous system. The zonal maturation of the lesions (immature myxoid cells at the periphery, mature fat centrally) recapitulates fetal fat development.