A study of spindle cell sarcomas showing myofibroblastic differentiation.

Five diagnostically problematic spindle cell sarcomas showing invasive character, cellular pleomorphism, and high mitotic rate were studied clinically and histopathologically by conventional light microscopy, immunohistochemistry, and transmission electron microscopy. They showed varied clinical courses, with two causing death within 5 years and three showing recurrent and metastatic behavior. All lacked a clearly defined line of differentiation by conventional light microscopy. By immunohistochemistry, all were positive for vimentin and alpha-smooth muscle actin; in addition, one showed focal S-100 protein positivity, and one stained for desmin. All were cytokeratin negative. By electron microscopy, the great majority of spindle cells in all cases showed abundant rough endoplasmic reticulum and fine myofilaments with focal densities; collagen secretion granules were also found in all cases but in fewer cells. The fine structure and immunophenotype were considered consistent with myofibroblastic differentiation; these tumors, therefore, were designated as sarcomas of myofibroblasts or myofibrosarcomas. The suitability of the alternative diagnostic label of myofibroblastic or matrix-secreting variant of leiomyosarcoma is discussed. Comparisons with similar tumors documented in the literature are drawn.