Ossifying fibromyxoid tumor of soft parts: a morphological and immunohistochemical study

A case is presented of ossifying fibromyxoid tumor of soft parts (OFMTSP) which occurred in the left cheek of a 59 year old Chinese woman. Histologically, the tumor was located in the subcutis with a fibrous pseudocapsule that contained discontinuous rims of mature trabecular bone. The tumor cells were small, round to ovoid with a uniform, round nucleus and a pale or slightly eosinophilic cytoplasm. Most of the cells were arranged in a random manner, some in a vague lace-like pattern. Mitotic figures were extremely rare. The stroma appeared fibromyxoid with scattered foci of mucinous lakes. Immunohistochemically, most of the neoplastic cells displayed immunoreactivities for S-100 protein, S-100 protein alpha, vimentin and alpha-smooth muscle actin; many tumor cells were positive for desmin. The tumor also showed stromal immunoreactivity for type IV collagen and was negative for cytokeratins, epithelial membrane antigen, glial fibrillary acidic protein, neurofilaments, muscle-specific actin, Leu-7, myelin-basic protein, osteocalcin and melanoma-specific antigen. The immunophenotypes expressed by the present OFMTSP seem to reflect two lineages of neoplastic cell differentiation, that is of nerve sheath and of smooth muscle; at the present stage, it is premature to diagnose this lesion as either nerve sheath or smooth muscle tumor.