MYOFIBROBLASTIC TUMOR OF SOFT-TISSUE DISPLAYING DESMIN-POSITIVE AND ACTIN-NEGATIVE IMMUNOPHENOTYPES

Myofibroblasts have ultrastructural and functional characteristics int ermediate between fibroblasts and smooth muscle cells, Previous studie s indicated that most myofibroblasts express actin and vimentin but no t desmin immunophenotypes. Two benign intramuscular myofibroblastic tu mors which displayed a desmin-positive and actin-negative immunophenot ype are reported. The tumors occurred on the back of a 45 year old man who had neurofibromatosis 1 and the thigh of a 37 year old man withou t neurofibromatosis. Both tumors were encapsulated and composed of sho rt intersecting bundles of spindle cells in a collagenous background. Although the tumors were cellular, nuclear pleomorphism was minimal an d mitotic figures were rare. Characteristically, most tumor cells were immunoreactive strongly for desmin and vimentin but gave negative sta ining for muscle-specific actin, alpha-smooth muscle actin, alpha-sarc omeric actin, myosin, S-100 protein, cytokeratins, and CD34. On electr on microscopy, the tumor cells were characterized by short spindle-ind ented nuclei, abundant cytoplasmic intermediate filaments, prominent p lasmalemmal pinocytosis and frequent cell coverage by basal lamina. Ag gregation of thin myofilaments with focal condensations was identified occasionally. Although the tumors showed distinct morphological and i mmunohistochemical features enabling delimitation from other soft tiss ue lesions, they may overlap histologically with myofibroblastoma of t he breast, lymph node and soft tissue, low grade malignant nerve sheat h tumor, leiomyosarcoma, cellular schwannoma, inflammatory fibrosarcom a and nodular fasciitis.