Ossifying fibromyxoid tumor of the retroperitoneum.

A case of ossifying fibromyxoid tumor (OFMT) in an unusual site, the retroperitoneum, in a 71 year old Japanese man is reported. The well demarcated tumor contained a large amount of mature bony tissue within the tumor. The tumor cells were round to spindle-shaped, and showed a high cellularity in some areas and were loosely arranged in other areas. Although the majority of tumor cells expressed vimentin, neuron specific enolase and Leu-7, there were no tumor cells positive for S-100 protein, myogenic or epithelial markers. We would like to emphasize that tumors classified as OFMT contain a wide spectrum of histologic features and heterogeneity.