Answer: Ossifying fibromyxoid tumor

Histology: This tumor consists of nests and cords of uniform round cells with bland nuclei suspended, in areas, in myxoid matrix and surrounded by a shell of bone, all features of the ossifying fibromyxoid tumor of soft parts.

Discussion: These tumors were described in 1989 by Enzinger, Weiss, and Liang, who reported 59 lesions involving the extremities followed by the trunk and head and neck. On follow-up, about a third had had recurrences and one possible metastasis was detected although there were no tumor-related deaths. It was not clear whether these tumors were of nerve sheath or cartilage origin, although most observers have subsequently favored nerve sheath origin. Most lesions express S100 protein, and their behavior is typically benign, although malignant variants have been reported. In contrast to epithelioid sarcoma, these tumors lack keratins. They certainly appear similar to myxoid chondrosarcoma but have more intricate vasculature and of course the marginal shell of lamellar bone is unique to ossifying fibromyxoid tumor. The lesional cells in myositis ossificans are myofibroblasts and fibroblasts and are thus spindled rather than rounded and these are S100 negative.