Myofibroblastic differentiation of a primary intracerebral sarcoma with gliomatous reaction

Microscopic examination of an intracerebral tumor removed from a 50 year-old man revealed a gliomatous rim surrounding a sarcoma. The sarcoma had central necrosis and vascular hyperplasia. Sarcomatous cellular elements consisted of medium-sized undifferentiated cells together with spindle cells associated with a loose collagenous matrix. Although grossly circumscribed, the sarcoma microscopically invaded the brain along an ill-defined border. Adjacent, and encompassed cerebral parenchyma had undergone gliomatous changes consisting of swollen body astrocytes with cellular and nuclear pleomorphism, hyperchromatism and cellularity, and the presence of multinucleated giant cells. The sarcomatous cells were unreactive to antibodies for carcinoembryonic antigen (CEA), keratin, glial fibrillary acidic protein (GFAP), S 100, actin and factor VIII although the endothelium and muscularis of neovascular areas were positive for factor VIII and actin respectively. The gliomatous rim and encompassed glial elements were strongly positive for GFAP. Ultrastructure of the spindle cell component, found only in the sarcoma, revealed well-formed myofibroblasts with dilated cisternae of rough endoplasmic reticulum (RER) and bundles of myofilaments with dense bodies. The medium-sized cells of the sarcoma were generally undifferentiated. However some of these cells had bundles of microfilaments with areas of condensation although these cells lacked the configuration and dilatations of the RER characteristic of myofibroblasts. Transitional cells between the mature and immature components also occurred. These findings suggest that this sarcoma originates from mesenchymal stem cells undergoing myofibroblastic differentiation.