The Comparative Role of Immunohistochemistry and Electron Microscopy in the Identification of Myogenic Differentiation in Soft Tissue Pleomorphic Sarcomas

Abstract

There is increasing evidence that histological classification of pleomorphic soft tissue sarcomas is prognostically useful, since a number of studies have provided evidence that myogenic differentiation is associated with a more aggressive clinical behavior. The aim of the current study was to analyze the role of electron microscopy in comparison with immunohistochemistry in the classification of soft tissue pleomorphic sarcomas. Thirty-nine pleomorphic sarcomas of the somatic soft tissues for which material for immunohistochemical and ultrastructural analysis was available were selected for this study. Cases were classified according to the criteria of the WHO classification of soft tissue tumors on the basis of the histologic appearance and of the results of immunohistochemical analysis, and then diagnoses were reconsidered at the light of the results of the ultrastructural analysis. The group of myogenic sarcomas included 13 leiomyosarcomas, 8 myofibrosarcomas, and 1 rhabdomyosarcoma, while the group of nonmyogenic sarcomas included 11 undifferentiated pleomorphic sarcomas/malignant fibrous histiocytomas (MFH), 4 myxofibrosarcomas, and 2 liposarcomas. Overall, there was a good concordance between immunohistochemistry and electron microscopy in recognizing myogenic differentiation in soft tissue pleomorphic sarcomas. Discrepancies included 1 case showing no immunoreactivity for muscle markers, which displayed ultrastructural features allowing reclassification as leiomyosarcoma, and 2 cases initially classified as undifferentiated pleomorphic sarcoma/MFH, which were reclassified as myofibrosarcomas after ultrastructural analysis. Ultrastructural analysis allowed the identification of pleomorphic sarcomas with myofibroblastic phenotype, a category that is not identifiable based on histologic and immunohistochemical profile. Notably, fibronexus junction was identified in tumor cells of 4 pleomorphic myofibrosarcomas, while 2 other lesions showed putative fibronexus junction structures, consisting of electron-dense straight fibrils adjacent to the cell surface, not clearly in continuity with cytoplasmic actin filaments. In conclusion, the results indicate that immunohistochemistry and electron microscopy can usefully complement each other in the classification of soft tissue pleomorphic sarcomas.