Ossifying fibromyxoid tumor of soft parts with stromal cyst formation and ribosome-lamella complexes.

Ossifying fibromyxoid tumor of soft parts (OFMT) is a recently named soft tissue tumor of uncertain nature. A case is described that presented in a 13-year-old boy as a discrete mass in the muscles of the lower abdominal wall. Light microscopy showed, in addition to the typical features of this entity, microcysts formed by accumulations of the myxoid stroma. Bone formation was lacking. Tumor cells were strongly immunoreactive for vimentin and glial fibrillary acidic protein and weakly so for S-100 protein. A few cells stained for desmin and alpha-smooth muscle actin. Ultrastructurally, there were abundant, patternless cytoplasmic intermediate filaments; short, poorly interdigitating processes; and discontinuous segments of thick external lamina. In addition, several cells contained typical ribosome-lamella complexes in small groups. Ribosome-lamella complexes occur in neoplastic hematopoietic cells but are uncommon in solid tumors, particularly those affecting the soft tissues. These findings extend the range of appearances described for OFMT, which is added to the list of tumors in which ribosome-lamella complexes have been demonstrated. The balance of evidence suggests that OFMT may represent a peripheral nerve sheath tumor of low-grade malignancy, although the picture is incomplete.