Plexiform fibrohistiocytic tumor. Report of a case involving preoperative aspiration cytology and immunohistochemical and ultrastructural analysis of surgical specimens.

A typical case of plexiform fibrohistiocytic tumor (Enzinger and Zhang) occurring in the skin and subcutis of the abdominal wall in a 7-year-old girl is reported. Preoperative fine-needle aspiration cytology revealed a benign lesion with fibroblastic-histiocytic features which also contained bi- and multinucleated giant cells. The surgical specimen showed a tumor with multiple small nodules within fibrous septa; these nodules were composed of spindle cells and epithelioid cells and contained scattered multinucleated osteoclast-like cells. The tumor cells showed ultrastructural and immunohistochemical features of myofibroblasts and histiocyte-like cells. Thus, there was an abundance of lysosomes, prominent filopodia and bundles of thin cytofilaments along the cytoplasmic border, as well as immunoreactivity for alpha-smooth-muscle-specific actin, alpha-1-antitrypsin and alpha-1-antichymotrypsin. Ultrastructurally there were tumor cells exhibiting features of histiocytes which also contained bundles of actin of smooth muscle type. The presented case of plexiform fibrohistiocytic tumor appears to be composed of a rather peculiar cell form, somewhere between myofibroblasts and histiocytes.