Palisaded myofibroblastoma. A benign mesenchymal tumor of lymph node.

We report 22 examples of an unusual and distinctive benign mesenchymal tumor arising exclusively from lymph nodes of the groin. The tumor, which presents clinically as a swelling, is composed of spindled cells arranged in solid sheets or short, vaguely palisaded fascicles similar to a neurilemoma. The spindled cells blend gradually with large mats of eosinophilic material that appear as thick bands, ellipses, or circular profiles, depending on the plane of section. These eosinophilic structures, which represent a highly characteristic feature of the tumor, contain deeply eosinophilic, collagen-rich cores surrounded by a weakly eosinophilic, actin-rich cuff. The actin within these eosinophilic structures is derived by coalescence of intracellular actin globules extruded from neighboring cells. In all cases, a thin, compressed rim of normal lymph node was identified. Immunohistochemical analysis indicates that the cells express actin but lack S-100 protein, synaptophysin, desmin, keratin, and epithelial membrane antigen. Delicate, linear striations were identified in only two cases by conventional histochemical techniques. The foregoing features suggest that the tumor is related to a myofibroblast or a specialized smooth-muscle cell. These tumors, therefore, probably arise from smooth-muscle-like cells, which are normally present in some lymph node capsules or stroma. Follow-up information on 17 patients indicated that all are alive and well without any evidence of recurrence or metastasis.