Myofibroblastic tumor of soft tissue displaying desmin-positive and actin-negative immunophenotypes.

Myofibroblasts have ultrastructural and functional characteristics intermediate between fibroblasts and smooth muscle cells. Previous studies indicated that most myofibroblasts express actin and vimentin but not desmin immunophenotypes. Two benign intramuscular myofibroblastic tumors which displayed a desmin-positive and actin-negative immunophenotype 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 without neurofibromatosis. Both tumors were encapsulated and composed of short intersecting bundles of spindle cells in a collagenous background. Although the tumors were cellular, nuclear pleomorphism was minimal and mitotic figures were rare. Characteristically, most tumor cells were immunoreactive strongly for desmin and vimentin but gave negative staining for muscle-specific actin, alpha-smooth muscle actin, alpha-sarcomeric actin, myosin, S-100 protein, cytokeratins, and CD34. On electron microscopy, the tumor cells were characterized by short spindle-indented nuclei, abundant cytoplasmic intermediate filaments, prominent plasmalemmal pinocytosis and frequent cell coverage by basal lamina. Aggregation of thin myofilaments with focal condensations was identified occasionally. Although the tumors showed distinct morphological and immunohistochemical features enabling delimitation from other soft tissue lesions, they may overlap histologically with myofibroblastoma of the breast, lymph node and soft tissue, low grade malignant nerve sheath tumor, leiomyosarcoma, cellular schwannoma, inflammatory fibrosarcoma and nodular fasciitis.